I'd have thought the simplest solution would be to not allow 'For Sale' postings on Mudcat but do allow someone telling everyone that they have put something musical e.g. Takamine EG-123 Guitar, on e-bay. That way Mudcatters are alerted that something, in which they may be interested, is available but all the details and process of buying is handled by e-bay (or Loot or whatever).
btw I think the above is a different category to advertising folk clubs and gigs.