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. 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.
This is honestly the best solution for your case. There's no need to implement such feature when this kind of activity already exists out there in the open and there isn't a net benefit of implementing such feature (which will require a considerable amount of work I believe). All you have to do is make sure you find someone that is neutral to both sides (but at the same time someone you both trust) and use him/her as a 3rd agent of the trade that will only release whatever he/she is holding if the transaction is made (similar to smart contracts but replace it with a man/woman).
I don't know if you're intending to use bitcointalk as a place to look for escrows but there are many reputable ones in here - check these threads: