This is a case of 'buy the seller' more than the watch. If they are 80% sure, but willing to provide a certificate of authenticity, the certificate isn't worth the ink it was printed with.
The case back looks fake to me, I can't imagine Omega would forget to put the hole in the letter 'A' in Seamaster. The 'waterproof' striking is also really bad looking, and I'd expect better out of them.
The band looks like junk too, but 'matches' the case well enough that I'm suspicious of it. The fact that the lugs are plated, but the case sides aren't is a really bad look I can't imagine Omega ever would have done.
The pics of the dial are bad, so I can't see very much that is suspicious there (due to lack of detail!), but there is a few things there that don't look right either.
I would personally pass on this one, both because of the seller, and the watch itself.
