Hi Bart,
welcome to a very addictive hobby - I’d suggest you skip on this one, though. Google “Omega 136.0098” and look at pictures of a few other examples to compare the lugs. This one has been heavily polished and if you zoom in on the edges of them you’ll see the gold plating has been polished off. It has lost its distinctive, sharp shape and the decorative thin layer of precious metal. The crown is also very worn.
By the way: The watch was sold on Catawiki only a few days ago for 260€, which is a much more realistic value for it in this condition (although I’d still consider that too much with a case this poor). Here’s the link:
https://www.catawiki.com/de/l/26879057-omega-geneve-136-0098-herren-1970-1979
Your seller is simply a flipper, trying to make 100€+ or so on you. Check eBay open auctions for those Genèves, they usually end between 250-350€, even if they’re quite nice. The classifieds of this forum might also be good to check out occasionally.
