Yes, there was a warning. The problem is that the seller could claim his account was hacked and get it reset by the Lisk team.
What I could do is to reveal my full identity to the buyer. The buyer would still have to trust me.
Just provide your mail, acc and passphrase to a trusted escrow and wait the wallet release. Than he'll provide them to the buyer and you'll get your BTC from the escrow. There is no other safe way for the buyer!