There are some major facts that maybe you are not aligned with.

In order to maintain it's 501c3 status, one of the restrictions is that it may not align itself with any commercial activities online.
Hence, links to commercial sites, buying and selling (trading is ok) are not permitted on the club forum.

Without those restrictions the forums could have been merged years ago. However, you then have to start restricting the topics of discussion also.

You didn't really answer the first question. If you walked into Walgreens and asked them to hang a poster in their window advertising CVS, do you think they would agree?