I'm not sure what you mean by 'dodgy' signatures.
I'm not going to attempt to define a British expression, hee hee, but I had an experience at the last Fest that's along the same lines. There was an autographed photo,
not DS related, that I coveted like you would not believe, but I thought the price was way too high. But I kept thinking about it so I decided to return the next day to make an offer. I noticed it was still there while passing by on Sunday morning (at nearly $100, no surprise there), but by the time I returned in the afternoon the dealer had left. Thank the gods and goddesses.
I wrote a friend (a mutual fan of the actor) about it, who only a few days later pointed me toward the star's own website on which he had written that this particular photo had shown up on ebay and that he had never signed anything of the kind-- it had to do with his contract with the movie maker-- and he planned to take action against the seller. Of course I was extremely relieved I hadn't bought the picture at any price. BTW, the actor has since begun selling autographs through his site, for much less, and though the pics are of a different role, I bought one. I think he's marvelous.
Now please understand that I'm not trying to pick on the dealer and the person that's guilty of fraud is the ebay seller; I only want to illustrate the point that autograph collectors must beware. I usually only buy DS photos and autographs from dealers that I know and trust, such as Jay Nass, but I found myself caught up in the emotions of the moment and should've known better. But DS fans are so lucky to have reference books and articles available to them as well as experts such as doombuggy and yendor, Dean and Bobubas, etc., to help us neophyte collectors out.