And there are still people who don't know what they have on a yard sale. Exhibit A 27 grams sterling for $7.
Or a flea market bourse! Ten years ago, I went to my first ever flea market bourse in Westend Ottawa. About 30K sq ft. of stuff. I went to this old mans miltaria stall, I noticed he had a SS Officers Ceremonial Dagger with chain, perfect. I asked Him, what he wanted for it, he said $75 Can., I bought it. Later, I took it to a well expert, he was astounded, showed me a auction catalogue, its value $10-15K US! Sometimes you can snag a good deal.
Asymmetric knowledge. At a ham fest (flea market for electronic gear), I found someone selling a box of old Popular Electronics magazines from the mid-70s. He wanted $15 for the box. I looked through the box, and I saw that it included the January and February 1975 issues, the ones that introduced the Altair 8800 - the first mass-marketed "personal computer". I bought the box, and sold those two issues on eBay for something like $200.