The only restriction is self-deleting, as if the topic starter of a thread delets a post they made, ALL replies get deleted. This happened a few times on threads that turned out to contain some good information in the past, so I no longer allow member-deleting. This unfortunately does also apply to the equipment forum, but like Randy said, editing your post to say "SOLD" works just as well.
The reasoning for not permitting replies on that forum is because the forum is only to be used as a sort of electronic bulletin board. Actual transactions that result from a post there are no one's business but the two parties involved, yet for some reason I see other online forums where two people will carry out an entire transaction publicly, mistakingly thinking that it provides some sort of guarantee and can hold the website responsible. This is not the case.