Whether you're a hardcore shopper who combs the racks of the Salvation Army regularly or rely on secondhand shops as a place to donate your old stuff, you may have wondered what it's like to work behind the scenes. Wonder no more: Mental Floss talked to several workers at thrift stores around the country about what happens to rejected donations, the coolest (and weirdest) items they've seen, and the best way to score an even better deal.

1. THEY DON'T KEEP EVERYTHING YOU DONATE.

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Thrift stores don't always keep all your donations, either because they don't have room or because items aren't in good enough condition for resale. But rest assured they try to responsibly redistribute or recycle what they can't use. Stacie Morrell, manager of Homeward Bound Pets Thrift Store in McMinnville, Oregon, tells Mental Floss that her non-profit has several destinations for unused donations. They take the best clothing to a local consignment store, generating another revenue stream; they give their "pulls" (items pulled off the racks that didn't sell) to the larger St. Vincent de Paul thrift store across town; and they send donations that aren't fit for resale to homeless shelters or a recycling facility.

Goodwill, where Morrell has also worked, has a different system. The stores send pulls and poor-quality items to Goodwill Outlets, where the stuff is set out in giant bins for customers to buy by the pound. That can mean gems for patient thrifters—the stores occasionally send donations to the outlets "raw," meaning they haven't even had time to open the boxes before sending them off. "It's like a treasure hunt," Morrell says.

For really unsalable merchandise, Goodwill sometimes finds overseas buyers, and they recycle what they can. If they can't recycle it, it goes in the trash. "They have huge garbage bills every month, into the millions," Morrell says.

2. THERE'S A MEANING BEHIND THOSE DIFFERENTLY COLORED TAGS.

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If you shop at Goodwill, you'll recognize the colored plastic price tags on all the clothing. It's all part of a system for rotating inventory and keeping the merchandise fresh. Most Goodwill stores use five different colors and rotate them weekly in a regular pattern: A new batch of clothing going onto the sales floor might get a blue tag, for example, and next week's merchandise might get yellow. Every week on a Sunday, Goodwill puts the oldest color on sale for 50 percent off to help get it off the floor. If you can figure out your local store's color pattern, you can predict when an item that catches your eye will go on sale. Tip: Try (nicely) asking an employee.

Other thrift stores use different systems, but they usually have some way of tracking the date on their wares. At Homeward Bound, the month the merchandise arrived is written on the tag. "We have a sign up front that says, 'Items this month and before are 50 percent off,'" Morrell explains.

3. YOU MAY HAVE A BETTER SHOPPING EXPERIENCE AT A SMALL, NON-PROFIT STORE.

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Small businesses can mean a more friendly experience for the customer. "Most of the small thrift stores will be staffed by volunteers, so they want to be there," Morrell says. A volunteer at Housing Works Bookstore Cafe, a secondhand shop in New York City, explained to us: "They're not in business to make money. They're a charity. … The people that work there understand that, that they're there to be helpful to each other and to the community."

Another benefit: Unlike chain stores, small stores may let you bargain. "If a person came up to me [with a pair of $15 pants] and said, 'I love these pants. I've only got 12 bucks on me,'" Morrell says, "I'd be like, 'Sure,' because that's 12 bucks I'd have in my hand rather than the 15 I didn't."

4. THEY MAKE A LOT OF MONEY SELLING ONLINE.

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Believe it or not, your favorite thrift store probably has a thriving e-commerce business. When donations come in, employees separate out collectibles, books, and other higher-end items to sell online. That means it's worth checking out your favorite store's web presence occasionally. Goodwill, The Salvation Army, and Housing Works all have their own online stores, and sell books on Amazon, too. In fact, Housing Works Bookstore Cafe makes the majority of their money selling online, our source says. Mom-and-pop shops, on the other hand, might opt for eBay stores.

"For non-profit stores, e-commerce has become an essential part of selling," Morrell says. "I know a number of small business owners that, if they weren't online, they wouldn't be making the rent."

5. SOMETIMES THEY FIND REALLY EXPENSIVE ITEMS.

It doesn't happen often, but thrift stores sometimes come across a rare item and make a ton of money. Morrell set up the e-commerce department for Goodwill of the Columbia Willamette, where in 2006 she sold a 1923 watercolor by American impressionist Frank Weston Benson, whose work is also owned by the Smithsonian American Art Museum. The price: $165,002. Morrell was sorting through donations one day when she came across the painting and instinctively knew it was worth something. "It was in its original frame, but the frame was beat up. It had been obviously in somebody's basement and the matting had gotten wet." The work was signed but she couldn't make out the name, so she listed it for auction on shopgoodwill.com starting at $10. The store was contacted by a relative of the late artist; apparently, the family didn't know this particular piece existed. After the watercolor was authenticated, the bidding soared until it reached the hefty sum.

6. THEY GET A TON OF FORMAL DINNERWARE.

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A much-talked-about 2017 New York Times article documented the unfortunate truth that millennials just don't want their aging parents' stuff, especially not their china, silver, and crystal. And that means that huge amounts of it go to the thrift shop. "Those are wonderful, beautiful things and they look great sitting on a shelf, but that's exactly where they stay. It's hard to sell them," Morrell says.

7. THEY'VE DEALT WITH A LOT OF BAD CUSTOMERS ...

In an article for Cracked, a former employee of a major thrift store chain writes that rich customers are often some of the worst. "The thrift store I worked at was in a really wealthy neighborhood, so obviously we got a solid handful of rich, bored housewives who'd come in out of idle curiosity for how the other half lives," she writes. "The wealthy customers would talk to me as if being around donated clothes meant that I was also some kind of discount, donated human. One such woman sneered when I told her an Abercrombie shirt was $2.99, because she expected it to be free, apparently. After I finished ringing her up, she stood by the register and pointed out every dismal aspect of our store like a judgmental stepmother."

The anonymous Housing Works source says, "I think the most horrible customer was the one when the cashier told her that we don't take Discover. She put her Discover card in the machine and said, 'Well, I'm going to try my Discover anyway,' and it caused the whole computer system to crash."

8. ... AND SEEN A LOT OF GROSS STUFF.

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Morrell has seen a lot of bodily fluids in her time in the industry. "I was working at Goodwill and I saw a mom grab her kid and hustle out the door. I thought, well maybe he's sick or something or she got an emergency call. I'm walking the floor and I go past the toy aisle, and the kid had pooped and spread it all over the floor."

A former Goodwill employee who did a Reddit AMA once found a rat at the bottom of a bag of clothing. "It made the entire back room smell, and we had to get rid of all the clothes."

9. THE OCCASIONAL BAD EMPLOYEE WILL TRY TO GAME THE PRICING SYSTEM.

"People think that we glean the good stuff off the top, but that is absolutely not true," Morrell says. However, while she emphasizes that the majority of employees are honest, she has seen a couple workers who see a treasure, furtively price it for less than they should, and buy it for themselves or to flip online. Usually, there are rules in place to prevent that: The manager prices everything when it comes in, and if an employee wants to buy something, someone else has to ring them up.

"Most people are fine with this, and if they aren't, that's a good sign that they shouldn't be there," Morrell says. "Our first job as employees and volunteers is to make sure the organization gets the most they can out of every donation."