I'm sure there are always circumstances such as mentioned previously, but I have had great experiences from Property Room. I have bought many pieces of jewelry from there/Rosdebor and I always take them to an independent jeweler to have a second authenticity report made. Most recently, a 2 carat diamond ring and a .65 carat diamond ring. On both pieces the "estimated value" listed by Rosdebor has been in the "ball park"
My recommendation is to be very careful about the clarity, color, and condition of the diamonds. Only buy if you can justify your purchase supposing you had return it and be penalized the handling fee. Watch a bid closely to see who else is bidding and how much they are bidding, and then search the internet for similar cut, clarity, quantity, and color of diamond jewelry selling for retail before you buy anything. If you can't buy it for at least 2/3 off retail price, then pass.
Also, don't be crazy enough to take deals that are seemingly "too good to be true" HELLO- they are.