I've been reading and posting to usenet newsgroups since the early 1990's and I suspect you are opening a can of worms. If you allow ads or for sale stuff, it won't be long before you will have mostly ads - and not even pertinent ads. You'll get all the s*x and other trash as well as ads for items that have nothing to do with your alt-topic. Some usenet newsgroups permit a "sort-of" ad, which is basically a for-sale url (web address) as part of their signature line. That way, you don't get the advertising "content" taking over your newsgroup, but those who have items to sell can reference their personal sales site on the internet. Then, anyone not interested in the sale can simply not click on or go to that sale site. I don't sell anything, but I'll put an example of what I mean with a fake sales web address. (the address won't work, if anyone actually tries to go there).
Chuck - my items for sale at
Someone can substitute a real site - maybe even an ebay entry and not annoy everyone with a bunch of selling comment. If someone is interested in the sales stuff and has a question, they can then email the poster to get further details. I do think you should try to discourage postings that are purely for the purpose of selling something. That's not what non-commercial newsgroups are for - they're for various interest group news and discussions. But, too many non-commercial newsgroups have been "infected" with so much commercial selling that they are a waste of time to try to read and get any enjoyment or real (pertinent to the topic) information. If you accept the idea of postings that are purely commercial, then that's mostly what you'll eventually get. Any selling, if accepted at all, should be incidental to normal topical discussions and should not detract from that. Therefore, the placement of the words FOR SALE or ADV or any such note in the content, though obvious, still detracts from the intent of the group. If you want a sewing-mach-embroider sales group, then start one that is specifically for selling - such as alt.sewing-mach-embroidery-for-sale. Anyway, that's what I think.
Chuck