The luckiest I’ve ever been was when I found an old pong console from the 70s complete in box at goodwill for about $10. It’s such a neat piece of gaming history, and while it’s usual price is harder to find than most other items in my collection, there are a few ebay listings for a little over $100.
Of course a close second was the copy of Big Bumpin’ on Xbox I found for $3. It had the original seal too so that’s nice. I mean sure the seal was torn so that means close to nothing but it’s Big Bumpin’ so who cares?
At thrift shops: I got an umbrella that depicts old newspaper comics. I also got a plushie of The Very Hungry Caterpillar. Oh and some very old tin containers for candy.
At yard sales: I got a shirt of Reptar from Rugrats, and a Pokémon Yellow cartridge that had an actual event Mew on it (as opposed to the glitch that let's you find one as a wild encounter). Unfortunately, my brother deleted the save file without transferring the Mew.