There are no deals in diamonds. You get what you pay for. Let's get that out in the open air right out of the gate.
If you are thoroughly educated and can confirm that what is being sold to you is exactly what it is, and the report is legit, the items match the report, and you can return for cash money if they are misrepresented, fine. But typically, you can get better quality goods outside of the district. There are a couple within that district that are decent, but most of them are not ground level. ID Jewelry, Brilliantly Engaged (Mark Turnkowski) and Steven Kirsch.
I do not buy within the district, usually. My NYC dealer of choice is actually out on Long Island: Good Old Gold. Worth the trip and every penny.