Some of the fakes are as good as the real thing. I bought a fake Omega Seamaster in Dubai-cost me £40- four years ago. It has kept perfect time since then-and even fooled a watch salesman from Prestons of Bolton until he examined it closer.
So... it didn't fool him then? None of the fakes are as good as the real thing. Full stop. I wouldn't fancy a fake watch working in 600mm of water, let alone 600 metres. Fine if you only want it for showing off down The Dog & Duck, but Omega Seamasters are professional watches for men who take part in adventurous pursuits... not for blokes who watch James Bond do them on a Bank Holiday from their couch.
I think the original poster is right in not buying from eBay. Get it looked at properly before you hand any cash over.