I don't frequent garage sales, but I do buy a lot of stuff that I intend to sell and not use. 100% of it is flyfishing related items. I look for good deals on things, then sell them for a profit.
I use that cash to buy more and sell it. I do this because I generally have an item in mind that I want. Once I've finally have enough save up, I buy what I want and put it to use in my fishing.
It's kind of like a hobby for me.

I don't really see anything wrong with folks selling stuff they buy at a garage sale. They paid for it, so they can do as they please with it.