Common sense should have told them if an item is listed $7000 below fair market value, something is wrong. There is no polite way to point out to them that greed got the best of them, its a basic tool of con-artists.
I feel really bad for them, but then again I've never had $5000 to throw around to buy toys like that.
At least they were bright enough not to give them the complete $5000 and "only" lost $2000.
This is also why escrow services exist, and if they had at least used PayPal protection, the first $1000 of that would have been covered.