Yes, there are fakes and frankens. And when a vendor goes for too low or too high a price, I suspect one or the other. Those who look before they leap and learn what to look for do fairly well. But if you are not willing to do your homework, you will suffer the consequences.
But, for the watches I pursue, I actually find relatively few fakes and frankens being sold on eBay (there are half a million+ watches on eBay ) ... maybe that's because I don't even look at South American and Eastern European vendors anymore. And I no longer look at vintage Omega mechanicals - I just don't see the need to invest the time to be able to evaluate them. Similar watches can be had for far less money.