I've never seen live fish at a garage sale, but have found some good bargains. When people quit the hobby, it's usually because their fish died, not because there is anything wrong with their equipment, so as long as the price is right, you will rarely go wrong.
Op shops and goodwill are great aparently, even though i havnt struck success!
I regularly go to garage sales, like today, but i dont like the quality of the items most of the time. Like today one 3fter was on sale for $15 (just the glass tank) but it had duck tape around the corner???