http://walletdvl5avrmoxk2fwsf5b76zkmbkoo3r6kqeesq5hsgnq7xhbciad.onion/how-to-check-wallet.dat-is-real-or-fake
The rules are as plain as day: purchase only from sellers with a positive story, require time for verification, or buy using escrow. Basically, scammers do not let you take your time to check a file, they may haggle or sell several files for the price of one, but ... as practice shows, when they get the money, they disappear.