Frankly, I'd be very wary. What is the difference in price from buying from a reputable grey market seller? A reputable grey market seller would have boxes and papers, just not necessarily stamped papers (although often they are).
For Omegas, how much are you really saving? A grand? Keep looking--deals can be had on Omegas with full kit pretty easily. Too much risk as to provenance and whether it is genuine, but that's just my opinion.
Hell, I know an AD in Europe that'll get you an amazing price once you factor discount, exchange rate and VAT refund in.
so grey market seller do come with stamp these days? what's my best source for Omega