I am concerned looking at the Omega watches on ebay, people seem to be bidding hundreds/thousands of pounds for what are clearly fake watches.
Please make sure the watch you purchase has all the correct cards and if possible receipt. It is possible to purchase a very good looking fake for £30 if you know where to look. I have seen people advertise what is meant to be a water proof speedmaster etc and they then say "please do not wear this watch in water as it is a classic".
One very big bit of advice. If you see a watch you like, get the serial number from the seller,
Then go to the official Omega site you can send them a email with the serial number and they will tell you and send a picture of the watch that number matches for free.
Hope this info helps