Mudcat runs its own auctions too. I see nothing wrong with advertising equipment for sale, indeed it might merit an "equip" prefix. I also see nothing wrong with advertising club and other gigs, personal gigs Mudcatters' new CDs coming out etc etc.

The ability to trade is a vital part of any community, be it trading information or goods/services. I see Mudcat as a community, therefore I think advertising should be accepted. If it was an academic-interest site only, then I might have had a different view.