There's a lot of watches for sale on ebay. Some are fake, and sold as real by people who bought them as real and don't even know they are fake! Obvious clones to watch out for are Rolex, Omega, Tag Heuer, Longines, Oris.
This guide isn't really about spotting fake watches, there's a lot of those guides for you to read, but here's how to get a genuine mechanical (wind up or automatic) watch at a good price.
1. Look out for an unfamiliar name, WATCH the item (no pun intended), and in the mean time do lots of searches for the history of that name. I recently bought a Favre Leuba watch for £4.50. This mechanical watch keeps great time! I'd never heard of them. Imagine my surprise when I discovered that Favre is the second oldest watch manufacturer in Switzerland! Now recently re-launched it is a VERY expensive brand. Why is it worth looking out for an unfamiliar name! Simple. Why would the counterfeiters bother to copy them if few people have heard of them?
Any vintage watch of unfamiliar name should be worth looking at, especially if the seller includes good quality images of the front and back, and for preference the internal markings! However occasional sellers of watches, like myself, do not have the equipment to remove and replace the back of a watch. Points to look out for are SWISS MADE as distinct from SWISS MOVEMENT on the dial as these are two different standards for the internal workings. Do a search to understand these terms more fully. SWISS MADE is the best quality although SWISS MOVEMENTS are also very good indeed.
2. Look at the feedback seller has. Look at the items sold carefully. If there's a lot of famous watch brands and it's not a jeweller then ask if there is a return policy if not stated. If it's just a one off sale from someone who sells a variety of stuff then it may well be a bargain in waiting if the seller is clearing out unwanted items.
3. Study the pictures carefully. A good seller will try to include pictues which are as clear as possible. These will give you a good idea of the general condition of the watch and study the pictures of the watch backs carefully, and compare against other watch backs from sellers of watches with high feedback. Certain watches should have type and serial number somewhere on the case.
4.If the watch is claimed to be of recent manufacture(eg within the last five years) is there any supporting documentation? If I were paying a lot of money for a quality watch from a known manufacturer I would certainly keep the receipt, instruction manual and any authenticity certificates that come with it. Rolexes, Omega and such like can cost £1000's of pounds and if there is no supporting documentation, where is it?
5. Don't ignore other countries of manufacture than Switzerland. Seiko watches from Japan, especially the special editions, can be very collectable and reliable. There are also a lot of Russian mechanical watches avalable, although the quality of these can be very variable. Out of three I owned one has been excellent and is used daily by myself or my wife over 2 years, one had two of the luminous markers drop off and one had the winder come lose requiring repair.
6. Is the watch working and does it keep time? Any good seller should have tested the watch for at least a week of continuous running, both on and off the wrist. I set mine against a good quartz chronograph and see how much they gain or lose over at least a week. Don't be surprised or disappointed if there is a little variation after seven days. Unless the watch is described somewhere on the case, documentation or listing as a chronometer this is normal. Certified chronometers should have an individual identification number and it may be possible to check this with the manufacturer to identify the model and type. In some instances the watch you are thinking of buying may be 30, 40, 50 or more years old. The fact that they are still running is a testament to the quality of the design and manufacture.
7.Expect to have a mechanical watch serviced occasionally.
8. Lastly, decide your budget and stick to it. If you don't win the watch you want this time, others will come up eventually. A good quality mechanical watch should last you for years of normal use. But do keep a cheap quartz one around if your planning on doing some diy or heavy gardening!