Recently there has been discussion about breaking rec.photo.marketplace up into sub-groups, dealing with 35mm, medium-format, large-format, digital, video, lighting, etc. The problem that those proposing the division are trying to solve is that they find it difficult to wade through irrelevant messages. Simply dividing the group will not solve the problem, since many people will have a collection of items for sale, and expecting them to post to each individual newsgroup is not realistic; they will simply cross-post to all of them, and probably result only in an increase in irrelevant news due to an increase of finger-pointing messages.
The best idea I can think of is to encourage people to use descriptive subjects in their messages. The redundant "photo stuff for sale" does not help.
So, please, if you post a "for sale" or "wanted" message to rec.photo.marketplace, please follow this or a similar format in your subject line, and tell your friends to do the same:
Start with FS: or WTB: (that's "for sale" or "wanted to buy"). List the categories (35mm, medium-format, meter, literature) and/or major brand names (Graflex, Nikon, etc.). For larger collections, list the rough size (10, 20, 100 items). For a single item or two, list the price if you feel so inclined. Feel free to use adjectives such as "Mint" (as in "Mint Leica 1") where they are informational.