You might want to post info like diameter, depth %, crown height %, pavillion %, Sarin info on angles, color, clarity, actual carat weight, and pricing info for opinions. Folks here just can't advise you without more information. You want to make sure you are getting true value for the money you are paying.
I have no personal experience with Diamond.com so I cannot say if they are reputable or not. Maybe others who have purchased from them will comment.
I would strongly suggest an independent verification of any diamond you purchase, for your peace of mind if nothing else.