Proving ownership is one step but the actual owner, with private key in hands, can use it to broadcast a transaction and move bitcoin in that wallet (includes that address) to another (other) address(es). This first step is not enough to prevent scam in a trade if there is no Escrow service is used between to sides of a trade.
I'm only speaking of proving your wealth. Like when you prove you have stocks or bank account, you can transfer the money right away