Yes, it is a difficult field with genuine autographs on various items.
To get an autograph personally by Mark or other bandmembers is a very thrilling experience in the view of a fan. No question. Everybody know´s that, here.
But to sell autographs without clear proofs can be a very difficult adventure. The FBI says, that more than 70% of the autographs by sport artists (American Football, Baseball, Basketball and so on) are FAKES.
It is the same case with autographs by Rock-Bands, Singers and so on, especially most wanted artists, like Beatles, Elvis, Stones, Pink Floyd, Metallica, AC-DC, Queen and of course, Dire Straits with Mark. The so called "Certificates" which are very often used by US-Sellers, are not always clear, because they are also very often not genuine, FAKES.
So, if the seller has some more clear proofs of authenticity, like photos from that concert, the original ticket with the original invoice, photo-proof which shows the person, the poster and the DS-bandmembers, it will be more easier to sell it. It makes the whole thing more trustworthy. I don´t want to judge his the price-offer, but without clear proofs of authenticity it does not have any success.