Use a third party escrow who also know the things about property or whatever industry you are trading. You will send them the coins, they will hold the coins for your and the other guy. The escrow guy will monitor the activities from both of you and the other guys. Once you two agree and satisfy with the deal then he will release the coins to the other guy or he will give you your coins back.

There are no such lock system because who know you could misuse it. After having everything okay you still can sent those coin back to you instead of paying the seller.