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.
If the two parties (the seller and the buyer) know about multisig wallet, they can make use of it. It can be 2-of-2 multisig in which the buyer and seller will open such wallet. The buyer will send the bitcoin to the multisig wallet, both of them will need to sign any transaction on the multisig wallet before any payment (only one person will not be able to access or move the fund to another wallet). Onces the selling is completed, the fund can not be accessed by the buyer again, although yet can not be accessed by the seller too,but the buyer will sign the transaction which has been already signed by the seller to move the Bitcoin into sellers bitcoin wallet address. Using an escrow can make it 2-of-3 multisig which is better if the escrow provider is trusted.
Although, custodial service is good but t can be done in a noncustodial way too which is best for privacy.