If you go down the ebay route, some advice:
1. Decide on one or two models that you like and study them intensively. Be aware that as you learn more, your taste may change!
2. Don't waste your time with business sellers.
3. Don't waste your time with Buy It Now listings.
4. Don't look for a cheap watch that needs work doing. Look for one that appears all original and in good condition.
We were all beginners once and we all understand the difficulty and stress in finding that first vintage Omega. If you are careful and patient, you will find one to enjoy for years to come.