There are some really unusual fungi that can be picked up at the right farmers markets. I've never seen someone take along a mushroom brush to use on them though. Usually they are pretty clean and it is easy enough to see what kind of quality they are.
If you want to buy mushrooms at a farmers market, just try to find ones without blemishes and which are nice and firm and clean looking. Slimy or limp mushrooms are usually old or haven't been stored properly.