Usually the managers set the prices. From one thrift store to another may have a big difference in price. Worthless walmart bikes might have a 50 dollar tag while a Trek 1000 will go for 20 bucks. As soon as I saw that Trek, I locked my hand on it and didn't let go until I got to the counter. They said it was cheaper because it had flat tires.
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Some people will watch you in those stores and grab what you were looking closely at. They buy and sell for a living, many times on CL with a 200% markup. They aren't sure if something is worth some profit until someone else shows interest.
I always look for boxes of parts too. It's like finding treasure sometimes.
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When a store gets nearly full, they mark down everything to get rid of them. Keep track of what store seems overloaded, especially in spring when people clean out garages and storage.
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