Is this a genuine Ref 166.0257 ?

It's genuine but the case is very, very worn. PASS.

Follow recommended ebay listings thread here they are vetted and many new members buy their first watch from those listings.
Also Private Sales thread here. Better than fishing on your own on eBay.

I hope that is seriously cheap. Like 2 figures.

You are right to move on. Those use the 1010 movement, which is so plain that many would think it were fake. Maybe that is why he wont show it. Or maybe it is full of rust. There are exceptions but as a rule of thumb, any Omega introduced* from around 1972 to 1985 is visually disappointing and feels decontented in many ways, this is one such model. You also need to avoid gold plated cases with damage like here as it will look tatty by eye can can't easily be improved.

*Before anyone spits out their cornflakes I don't include the Speedmaster Pro which continued as it was with minor changes in this period, not all the changes were improvements though it must be said.