The sound of stable doors shutting is thunderous, and I really do sympathise.
But, .....
Never return original documents - if he has problems because of that... it's his problem.
Do all you can to find as much detail as you can about the seller - you knew this was an iffy site.
Always pay on a "secure" method of settling (ie PaPal or you credit card - both have money-back facilities in cases of a scam.
Keep copies of every fragment of communication you ever sent about the transaction - it's easy, never throw anything away!.
If you havee doubts about the seller/site, never spend more than you can afford to lose.
Obvious, I know, and means fighting temptation, but use reputable, traceable, sites (with a good track record hear ?)
Finally.... if it sounds too good to be true, never forget there's a chance it might very well not be.