E31Wiki is not some sort of market place. If you have or know a person who has a car or parts for sale, don't post it on E31Wiki. The same goes for a person who is looking for a car or certain parts. By the time someone reads the page, the car or parts may have sold a long time ago. The information on E31Wiki is meant to be long-term. Short-term selling information has no place on E31Wiki.
There are some exceptions though. If you provide or know a long-term commercial service that is directly targeted to and in the direct benefit of the E31 community, you may post a link to its website – or even add an appropriate article with a description of the offered services. Do not add individual items or services for sale. Leave that to the seller's website. For example, if you sell a single car as a whole or even as parts you, do not post it on E31Wiki but use the classifieds sections in the various E31 communities because you cannot guarantee long-term availability. If, on the other hand, you provide a breaking service with new E31s coming in every once in a while and thus have a more or less continuous supply of parts, the service is considered long-term and of value to the E31 community. This policy is not to allow only a small elite group to post commercial information while the rest may not, but wiki software is simply not suited very well for this kind of short-term activities. Once a page is created in a wiki, it cannot be completely removed. Its history will always be visible and short-term information may render the search functionality useless.
It's not difficult to distinguish between both kinds of commercial services, so use common sense. In case of doubt, contact one of the administrators. Any commercial message not meeting this policy may be considered spam. Repeatedly ignoring the policy on commercial postings may lead to a suspension of your user account.