So you doubt he actually owned this?
I think you can safely say whenever you see a signature on any guitar, it wasn't owned by Mark. Played on for a little bit, at most, but never truly owned it. Usually associated with auctions, fans seeking autographs and random guitar giveaways. I, for one, hate signatures on a guitar, to me, it actually makes it less desirable... And I know I'm not the only one.
In my opinion, this guitar shouldn't go far above the street price of an average Les Paul. Mark's signature is on a removable part anyway, the picture is not that amazing, to be honest, and it's a Les Paul with Grover tuners and domed knobs. At least the top is plain! So, if the seller is too demanding about it, I would give it a pass.