I used Blue Nile to buy my wife's engagement ring and was very happy with the service/quality. The online customer care agents actively pushed me to buy the lowest quality diamond that would appear 'best', as there are many levels of quality that can only be seen under a loupe by a jeweler. All of these grades will appear to be equivalent to the untrained/unaided eye.

She needed to get the ring resized later and it was very easy. Returned via FedEx and had it back a week later.