Q:

Is it possible that one knows how much BTC is in wallet.dat without knowing password?

Someone says that he or she wants to sell his or her wallet.dat which seems to contain a lot of BTC.
He shows screenshot of wallet software containing many BTCs.
He says that when I place wallet.dat in the wallet software location I can see how much BTC in contains, but it needs password to transact.
Is it possible?
Otherwise, is he trying to cheat me out of money?

A:

is he [or she] trying to cheat me out of money?

Yes.
Even if you had a copy of their wallet, they have the private-key and can control any funds that appear in your copy of the wallet, now or in future. Even after they give you the password and even after you change the password.
What they are probably giving you is a "watch-only" wallet - often used by conmen to deceive people. The password to a watch-only wallet does not let you spend the money in it. 
There's no need for a genuine seller to use this convoluted method of selling a wallet.dat file.
If they really wanted to sell Bitcoins they would  have you set up your own wallet without their help and have you tell them your bitcoin-address so they can send the bitcoins from their wallet to yours (note that the underlying reality is more complex, wallets hold private-keys not bitcoins, but this is the normal way to sell Bitcoins).
There is never a good reason to exchange wallet-passwords or private keys. You should never get involved in any arrangement that involves doing this.
You should do your own research on Bitcoin first and never accept advice or assistance from people who show such deceitful intent or appear to be trying to confuse you.

