I can relate in kind of a tragic way. Just a few weeks ago my cassette case got stolen out of my car (while I was parked at Earthfare of all places). 36 cassette tapes gone, tapes that I had as far back as middle school. So now I’m replacing them. If I were smart I’d do the flea market thing like you’re doing, but instead I’m buying them on eBay for two or three bucks a pop. Still pretty cheap though.
Man, I got a big tote bin in the garage full of cassettes I can’t bring myself to get rid of. I listen to an old iPhone I loaded up with just music and plug into a cassette adapter.
I understand cassette tapes are all the rage these days. [Which has raised their price from nothing...to something]