There have been many threads on this in the past, run a search on Advanced Search and just put in 'ebay' and you will find many posts on what people's thoughts on eBay are. I think it is safe to say the general consensus is that you really need to do your homework before dropping a few thousand on an eBay ring. Not all sellers are frauds and some just want to get rid of an old e-ring or a painful memory, but it is hard to know just what a 'great deal' is vs a piece of junk someone is overselling. There have also been threads on eBay fraud about people posting the same diamond twice , or people receiving cubic zirconia's and then fighting about return policies etc.
I would suggest that if you see a stone you really like on eBay, you email the seller and ask the following:
1-History/reasons for selling (e.g. are they are dealer or just Jane-Doe who wants to sell her old ring)
2-What sort of certification, and when was the ring last certified. If it has a GIA or AGS or similar lab cert that is a good sign, but if the cert is dated 1980, then the ring/stone probably needs to be re-certed to compare.
3-What was the apprasial value
4-What are specs such as table, depth, color, clarity, size/dimensions, pavilion and crown angle--these are all crucial to know the cut of the stone (not color and clarity but the others)--you may be getting a great deal on a dull, lifeless, horribly cut stone.
5-How will they accept payment, via escrow? This is the best way to cover yourself or via your credit card (not via PayPal but through your CC directly) where you can dispute the charge if things don't work out
6-What is their return policy? Some sellers if it is just JaneDoe trying to get rid of an old ring, may not have a return policy, you should take this into account because if the diamond/ring does not check out, you are completely out of luck. A return policy should be at least 7-15 days, the more the better.
If you do purchase, take that puppy IMMEDIATELY to an independent appraiser to get the reports necessary to determine just what you got..a great deal or a dull stone. If it doesn't check out, return the stone. Hopefully there is no hassle.
Personally, I have seen some great deals on eBay BUT I have also seen great deals online at reputable vendors who make a living selling nice stones to nice people. So don't be so quick to rush to eBay for a good deal when you can do a little research online and find probably a similar deal with a little more peace of mind.
Hope this helps..good luck. Also I know two of our members on this forum were recently selling rings on eBay...hopefully they will post if the rings are still available so that you can see if they are within your specs. Daisy was asking around 30-40% of appraisal price for her friend's 1.10 carat round stone and Jill was asking around 80% of appraisal for her 1.58 carat princess cut stone. Both had some form of paperwork.
Daisy and Jill..are you around?