I think you should still allow non car stuff to be sold. 
Surely you can appoint a moderator of the for sale section?
I do think it is a benefit to all members to give the forum a chance to buy something from someone they may have a bit more knowledge about than a complete stranger. 
An idea might be to either Charge a nominal membership fee (say £10 a year) which brings amongst other benefits the ability to sell stuff on here. I certainly wouldn't mind paying a nominal fee for a few extra benefits.
Or you could limit it to say having a certain no. of posts in the post count (say 250) if you don't want to charge. 
Or even a combination of both 
