Before you buy your watch (Omega or whatever) for this kind of money, you first spend a lot of time searching for the right model. When you found a watch that is suitable for you, you have to look around for the best prices/service/guarantuee etc.
This buyers guide is just meant to be a little reference for you for buying the watch for the right condition, a good service and ofcourse the right price. You can also skip this story and select 'Dealers' from the main-menu and find out yourself, but I advice you to read this text first.
Once you found the watch you wanted or the watch with the right price/value rating, you have to do some research on prices. If you are buying a new watch, there are ofcourse not as many things you have to pay attention to as when you buy an older/vintage watch.
Let's say, you are buying a new watch. Make sure you buy it at an authorized dealer who can give you the completely filled out guarantee card and who is able to take it in for service once in a while (only mechanical watches). At this authorized dealer, it is hard to get a better price than the price in the catalogue. If you get at least 15-20% off the price, you can call yourself lucky.
For used watches, or vintage if you like, there are some things you have to pay attention at. The following tips are from magazines, internet and actually real people (I assume you already have the right watch in mind, if not, take another good look on this website ;-)) :
1. Before you go out and buy a watch, try to get some literature regarding the watch you are planning to buy. For Omega watches, there are plenty of books, magazines and since a few years, the world wide web! My site is hopefully one of them. For books on watches, try http://www.amazon.com . They carry a lot of watchbooks and watch guides.
The Complete guide to watches (currently vol 19) is a nice reference for vintage watches. It is about 35 USD and almost available at every online bookstore. For new watches, the Armband Uhren Katalog 2000 (German)/WristWatch Annual 2000 (English) is a very nice source. It cost about 20 USD and carries all nice (mostly mechanical) watches from almost every 'descent' brand. You can also try the catalogues from auctionhouses like Henry's or Sotheby's. The prices in those catalogues don't reflect the real prices in general, but should give you an idea. Also give http://www.ebay.com a try. Just fill in the brand of the watch you want as a keyword and you will probably get a list of watches and prices. Worth checking out!!
check if the serialnumber is engraved. The serialnumber is on the back
of the watch, inside the watch or on the other side of the caseback. If
you want, you can verify the serialnumber by Omega in Switzerland ( http://www.omega.ch)
3. Check on the details of the watch. Does the watch have the original crown (signed with symbol since 1950's), is the dial clean and without too much spots?, is it signed?, are the hands original?, no cracks? Try to get a picture of the original watch in catalogues, magazines or old watchbrochures. If you are planning to buy an Omega Speedmaster (Professional), take a look at the pictures in the "Changes through the years" page for details.
4. Let the watchmaker open the watch so you can take a look at the movement! Is it signed, does it have the calibre number written on it? On some watches (Speedmaster Pro), an individual number, also known as 'serial number' is written on it. Study the movement very carefully, also when you don't have much knowledge on it. Look for rusty spots, scratches etc.
5. Always ask for the original accessoires like the original watchbox, straps with original clasp or stainless steel bracelet. These items surely do increase the value of the watch. If the watchdealer does not have them, try to get something of the price and try to get them (accessoires) at watchfairs or via the world wide web.
6. Good dealers will give you guarantee on vintage/used watches for a specified period and offer repairservice for the watch bought by them. For intance, I got 1 year full guarantee (not on the glass and leather strap ofcourse) on my 32 year old Speedmaster Professional. Also ask for guarantee at online watchsellers. If they sell online, that doesn't mean that they can not give any service on a watch!