Stritchy wrote:But then we might as well make it open to non-members

But look how many non-HOC members register on the forum, and their first (and only) post is an advert for something to sell ! They don't join the forum for any other reason.
Most HOC members who place an ad have already been forum members for some time and made many posts on other varied subjects. I expect anything they might put up for sale for a friend is a genuine offer by the HOC member to advertise it for them, with the hope of it going to a good home. It may also be the only place it is offered for sale.
Many non-HOC advertisers have mentioned that the item is already on ebay, with a link to it, (and it's probably in Trade It, in which it is free to advertise too) but not MCN as that's very expensive. They are just hoping to make a sale, and are not in the least bit bothered if the item goes to a good home, or even interested in the HOC.