I understand there is a feedback system but it's only effective after someone gets scammed. I don't see a good reason to allow a normal user to lock topics. User input and sellers reactions may prevent a lot of scams.
A locked thread usually means the topic is done with. When talking about sales, locked threads should mean the seller either doesn't sell anymore or the goods got sold already.
If you want to be on the safe side, don't buy anything if the sale thread is locked. Easy solution.
No need to remove a feature that got it's use over this.