Should I return this watch?

Montblanc Heritage Automatic, reference 11994

I bought it on ebay for $1600 ($1740 after taxes) and received it yesterday. Advertised as "2020 model, condition of watch overall is excellent, since watch is barely worn. Very minor hairline on watch bezel, but glass is pristine with no marks. Band is also pristine."

What was not disclosed was a moderately scratched caseback (which I could have possibly overlooked) but on close inspection, it appears that the watch might have been opened. I haven't held it long enough to gauge accuracy or ability to hold power reserve. I'm concerned whether the movement has been tampered with. The watch is out of warranty.

Am I better off returning it or is it worth keeping if it proves to run well?

If you can’t live with it like that then I suggest a return. It will be all you see and will be a disappointment. If you can then I wouldn’t bother.

I believe this is the scratch on the edge of the case not the caseback itself. It does look pretty bad to me but it’s a tough call if one can live with something or return. Especially if it was purchased at a good discount or if it’s a harder to find watch. If the scratch was visible from the front it would be an easier call.

With a little laser you can restore it to new condition, at a cost of 100-150 Euros.
It depends on whether you are satisfied with the price paid.