This article is based on material authored by members of the news.newusers.questions Moderation Board and nnq-workers mailing list.
Most newsgroups don’t allow any kind of buying and selling.
For general sales, you may want the newsgroup misc.forsale.non-computer or a group that ends in .forsale or .marketplace.
If you want to find a place to advertise or sell items that are more specific, you should not post them to any and every newsgroup available to you. Many newsgroup hierarchies have separate groups for buying, selling and advertising. Newsgroups may have special restrictions as to what types of ads can be posted to the group, and how often they can be posted there.
Your best bet is to ask, in a suitable newsgroup, whether advertisements are permitted in that group and where ads or sales posts could be posted instead. If you’re polite, the readers will be more than happy to help you!