No, that is not a particularly good OEC. That site takes a 25% commission, so never buy through that site. You can bargain more on price if you go directly through the vendor.
This seller is a well known, trustworthy PS member who is an avid collector and sometimes sells things she decides not to use herself. I don't know if this stone is still available, but it is $3000 and looks a little better cut than the one you linked. It's number 3 on the bottom left. You can also ask her if she has anything else she might part with in that price bracket.
loupetroop.com
I'll see if I see anything else that looks like a possibility.
