Let's even assume there's Bitcoin in the wallet and the seller gives you the seed prase, why is anybody buying a wallet? Why not just download a wallet and buy Bitcoins? Better still, why no just buy the coins from whoever wants to sell?
They are selling known locked Bitcoin Wallets convincing the buyers there is a hope to recover the Password.
We have some of the most knowledgeable people on the Bitcoin Talk forum and unless the Wallet owner knows at least part of the Password or how many characters the Password has, it is almost impossible if not impossible to crack the code even with the best computing power available today.
Otherwise most of the Bitcoin Talkers who lost their Password would of recovered it easily and we would know of a way to crack it. Sad truth is there is pretty much no way of doing it if the dat file has a strong enough Password.
And I would assume that some body would not be stupid enough to sell a BILLION Dollar dat file for cheap before trying to get the best hackers in the world to try and do the job. It would be utterly stupid to decide to sell it instead.
Anyway. OP is just wasting time in false hopes unfortunately.