The key is finding out if it's 'once fired' brass, which is usually range pickup brass, or 'used' brass... which is something else entirely. Ask questions... if they hem-haw around, or won't answer your questions... leave it.
I've sold brass meself... including .38's and .357's. Having to look at myself in the mirror every day, I sold it as reloaded brass, not specifically 'once fired,' because I know I'd reloaded it all at least once before, and priced it accordingly.
Handgun brass is very different than bottleneck rifle brass... it is a very rare occasion that I would buy someone else's used rifle brass, even if it's once fired.