Yes, I think some sort of 'classifieds' section would be a good idea. It doesn't just have to be for sports/fitness equipment, you could allow anything (so long as it vegan) that site members have for sale or want. I have a few spare bike bits I'd be happy to give away for the cost of postage and/or a charity donation.
I don't think the site has to worry legally, as any buying/selling is between individual forum members.
For protecting members, hopefully most people can check details of whether it looks dodgy or not. eg if it's someone who has been around on the forum for quite a while, and got a reasonable number of posts, then you can probably trust them. Plus you could check whether they had bought/sold anything previously, and if there was any problems.
Maybe the section could be only visible to registered / logged in members?