There are two safe options.
The first is to spend hours reading up all you can find about old Omegas - I'll suggest you go to the earliest page in the "vintage omega watches" and work your way through the years. That'll enable you to make solid, qualified decisions on eBay and elsewhere.
The second is to watch the sales forum here. If a watch is incorrectly described it tends to get pointed out rather fast. Look at some of the older FS posts to get a feel for what sells quickly and at what prices...
Welcome on board, by the way
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