The old adage of "caveat emptor" (buyer beware) is one that is important when buying vintage watches from any venue, not just Ebay.  Several things to look out for. With counterfeits look for for poor printing on dials and look for sharp corners and edges on watches that are supposed to be 50 years old. Also keep an eye out for Swiss watches that do not say "SWISS" on the bottom of the dial. This is one indication the dial is refinished and "possibly" fake. (not always of course).

Always look at ALL sides of a watch. Look at the back and the front. Look at the sides. Sellers who sell watches with many pics showing ALL sides of the watch are more likely to be sellers you will be happy with. Also be aware of terms like "custom" and "homage" and "after market". While these terms do not mean a watch is fake they DO mean that a part or two MAY be fake or MAY be added or a siginificant repair of some kind. This will lower its value. So add that your equation when bidding.

Do not be afraid to ask questions. Ask the seller for more pics. Ask the seller questions. And by all means use paypal or credit cards. NEVER pay cash or check.

good luck!