Respectable is not the same thing as reliable. They are a perfectly respectable (sizable, providing good service, fast) business, just not a reliable one.
Yes, they know something about it: it is not what GIA would call H/VS2. The price of a GIA report is $100 + shipping and handling costs (and a 6-8 week wait), so if it were an H/VS2, they would send it to GIA and for $200 it would magically increase in value by $4000 or even $5500. Not a bad return for less than two months of waiting.
The problem is that you have no idea what that diamond is in reality. There are plenty of cases where EGL is off by 3, 5 or even more grades overall. And there are quite a few where they are within one or 2. As a consumer, your best guide is the price: if the dealer is selling it at the price of a L/SI2 (as in your case), that's what it probably is. However, that assumes that the dealer is honest - what if it is an M/I1? Would you be able to tell the difference?
Some more threads on the "why the lab matters" topic:
http://www.diamondre...appraisal-firm/
http://www.diamondre...l-value-in-egl/
http://www.diamondre...-round-diamond/
Lastly: yes, there is always a discount for cash. However, paying with a credit card also provides you with some recourse against an unscrupulous seller. Credit card commission is of the order of 2-5% depending on the credit card company; plus they pay at least 30 days late, so dealers will typically offer 3-4% discounts for cash; let's call it $200 in your case. Compare that to the legal cost to recover your money if something goes wrong, and decide the extent to which you trust the seller.
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