Read the Price Scope Tutorial and browse through the posts in the forum. eBay is a great place to pick up deals, but you'll understand why you don't want to go eBay for this purchase. It's all about Cut, Cut, Cut. And for the most part, eBay sellers don't deal in top quality levels of cut. Plus too, their prices are higher than comparable diamonds listed through Price Scope.
After you've read and learned, click on compare in the top menu bar of Price Scope. You can check inventory (it's always changing) and compare prices.
Check out the Price Scope Vendors.
White Flash,
Good Old Gold, USA Certified, Superb Cert, Affordable Diamonds and Trade Shop, just to name a few. Go with the one that guides you the most, makes you feel most comfortable, has the best return/exchange/life time trade up policy, etc. From the many posts I've read, the PS vendors are all reputable. They stand behind their diamonds. No one has ever said they weren't given their money back if they weren't satisfied. Several PS vendors will even ship direct to an independent appraiser prior to you having to pay for the diamond.
Good Luck and Happy Hunting!