I purchased a few Omegas from here. I purchased two off eBay from “reputable sellers” (the two I bought from had over 500 positive feedback with 100% positive feedback). I prefer purchasing through WUS. Check feedback and if they have several transactions and multiple recommendations from people chances are it should be fine. Many users on WUS also have eBay accounts but due to rising fees in recent years I tend to stay away from selling on eBay if I can avoid it. You can find some nice older examples here depending on what you’re looking for and prices are generally pretty fair.
If you’re looking strictly Vintage I’ve been advised that you will likely overpay for vintage Omega unless you know what you’re looking for and you know where to look. This site was recommended to me for vintage. Overpriced for sure, but they seem to have good selections and variety.