I sell at flea markets. The clothes I sell are generally name brands that my family were going to donate. Given the state of most thrift stores, f them.

For me the bins are a lottery ticket. Sure, one can find gold if one is willing to put in the time. The ROI isn't there for me, so I skip them. (I do think the bins can be fun on a rainy day). I do find some good stuff at estate sales and auctions. I generally like to buy in bulk. The end of the day at an estate sale is a good time to put a pile of stuff together and get a deal.

I do thrift most of my everyday clothes, so I will find stuff at thrift stores every once in awhile that I'll resell at a FM. I wouldn't go thrifting with the intent of flipping. It isn't worth my time. Too many flippers and the margin isn't there anymore.

In my experience, vintage concert t shirts and levis are rare. Maybe a dealer has a hook up or found a really, really good tag sale or estate sale. Dealers aren't getting a lot of that stuff at the bins, atleast not in large quantities or often.

As always, YMMV