Selling links in your sig, is it a good idea to promote on your forums?

Hi,

I'm just wondering if you set up a forums and wanted to get more signups, then would it be a good idea if you mentioned in your forum / signature rules that selling of sig links to others is allowed in your forums as long as they don't point to illegal, porn sites, etc. Or do you think this will make people not want to sign up because of you allowing them to sell their sig links.

I know that some forums don't allow selling of links and thought that even if they are not allowed, people seem to do it anyway and some of the forums probably will not find out if each member owns that site or not anyway. So I bet at least a few members do it on most forums on the web.

Just wondering what to do here you see, don't want to do something that could damage my forums, which is the reason I am doing my research here as I know that there are forum owners here and sig link sellers aswell.