but is there any way to find out if a tPF'er is trying to sell their bag?
Yes, the market plaza sub-forums. It's private so you have to sign up for it. But it's great because it's free!
I think you might be talking about if someone on eBay claims they are a PFer??
If so, ask that seller for their PF id and search for them
How do we sign up for it?
There are instructions on the general page, but you have to have been a member for 5 months AND have 500 posts
and must be approved, just meeting the minumum critera is not a guarantee you'll be given access