Some people just don't know what they have. In my opinion, it's their fault for not researching. I've seen Contax 645 kits for under a grand when the craze was at it's height and people were selling kits in bad shape for $2500.
Were you the buyer in those cases?
If not, how can you be sure how the transaction ended? Did you follow up with the buyer about the condition and to make sure no fraud was at hand?
"some people don't know what they have" is absolutely the case sometimes. However in my experience that statement applies more to truly obscure items which would require highly specialize knowledge, rather than a body which you can google or look at recent sales on eBay in under a minute of effort.
It's true that this could be a "lucky find". Sometimes rich sellers don't really care the price it goes for (though usually they care a great deal); sometimes less rich sellers just need to turn their item ino cash RIGHT NOW and time is more important than absolute dollar price. But often a deal too good to be true is just that.
Does it have a warranty? Has he tested it in all the situations you will use it in (see also: torger)? Does he have expertise to check the firmware, uv filter, front/rear lens element, lens alignment/sharpness, cf card and FireWire connectivity, etc etc?
Let me ask the more pertinent (to me) question: have you thoroughly considered if this system is right for your needs. If you stay in medium format then you will, without a doubt, spend more in thr long run on accessories, lenses, an other items than on this original purchase. Once you've picked a platform/system you are not 100% locked in, but the cost (in $ and time/effort) is high to change.
This may well be the absolute best system for your needs/wants/style, and this may be a very lucky legit sale. Or you may be letting the temptation of a "deal" blind your better judgement.
My suggestion is as it always is:
Find the right product for you (whatever it is), THEN look for a deal. Not the other way around.
I also (openly selfishly) suggest working with a good dealer especially on your first MF purchase. Sometimes its more expensive (it would be in this case assuming this item is legit and in great condition), often it's the same or less, but it assures access to testing/evaluation, expert advice, an easy transaction, a warranty, and someone to hold to the fire if you feel slighted in the least.