I doubt anyone's making a full-time living selling rap CD's off eBay. A side hustle is all it is. But it's definitely possible to list 100+ items a month and still work a full-time job.
As for traveling from store to store buying shit, that's the only way to go. Pawn shops are my favorite spot. $2-$3 per CD. Sure, they have a lot of crap mixed in, but if you have patience, you can come up on some gems.