Per my other comment here...
"I've been doing this for years, so I have experience. I also look each item up on eBay before I buy them. I filters the search to see what they recently sold for to get an idea of market price, or since I see a lot of the same things, know by memory/rough range. If something is scarce, I make up my own price and force the buyers to spend to get it."
I also sell stuff I find for free on the curbs in my city's recycling (I mainly do that these days). I did drop by 2 thrift stores on the way home from baseball practice today and found a used tennis racquet for $6 that regularly sells between $140-$200, and a sealed Lego set for $3 that sells for $65-$70. I was familiar with the Lego set and recognized the tennis racquet's value by eye since it was a good brand and oversized head. I then confirmed on eBay's recently sold search and made the buy with confidence. It's a simple process once you develop the eye and know how to spot the value in stores when most is junk.