I do gamble once and a while on eBay. For example, have a $20 small lot coming simply because it appeared to have an egg yolk Akro shooter in it, which might be worth $20 if it's particularly mint and the pattern looks as cool as I think it is.
Also sometimes I buy agates that I suspect aren't hand-faceted. About one out of four times they turn out to be the real deal.
My rejects I put in my big jar which I sometimes let neighborhood children take marbles from.
For me the anticipation thrill is almost worth about $20, so as long as I don't do this too often, it feels like part of the fun.