Ebay is the "new" forum for trading and buying, just like the old classified ad section of a newspaper, only it is online. Sure, you ought to do your homework on items bought from ebay, just as you would anything else. Unfortunately, Rolex and Omega too are highly copied watches. But there are plenty of legitimate traders out there and you will have to take your time to learn all the ins and outs.
The classified advertising section of the newspaper used the be the highest read and most followed section of the newspaper, higher read than even the front page. Trading with other people is important. So I wouldn't avoid eBay--most people are honest. But there are enough dishonest people that you can get burned, so doing your homework, looking for references, etc., is important.
I ran into a dishonest Rolex trader just this morning, in fact. He has a website, has several references from the Better Business Bureau, etc. And in fact, he even has a section on how to tell a fake rolex from a real one. He also carries a section listing Rolexes which were "stolen", supposedly to help owners get their Rolexes back, I guess. The prices are below normal retail and used prices, so the watches are supposedly a good deal. But the site is a fraud and he has taken money from several people I guess now. Everything looks legit and even helpful. Fortunately, this is the exception and eventually, buyers will catch up with him. But this doesn't help you get your money back. The only way to help yourself is to become familiar with the product and the market and the reputation of the person/site you are dealing with. Look for positive references. The best clue is to see when a watch is priced below other ones in the market--it's not the only sign, but sometimes saving a buck is not worth the heartache. Remember, everyone in business needs to make a profit. Best of luck.