Personally I am always very careful if the seller give me incorrect information. You can also check the “sold listings” on Ebay (normally that goes back at least a couple of months), and his feedback. On both I could not find any previous sales by this seller of an NI ELVIS-board.
Sometimes I take the risk anyway, but this is typically for cheaper items (so not 800$) and if the price is very good (this seems to be the typical going price for such a device, and there is another currently listed for 775$, so why take the risk with this seller?)
A more important question, are you really sure this is something you want? Personally, I do not like these all-in-one devices, because it makes upgrading one part impossible, and it is very annoying is on part of it is broken. I am also not a big fan of NI software. Off course, if only very little space is available, this can off course be the only solution…