When posting something for sale, have a concise but complete subject line. Come to think of it, this is good advice for any post. If it is fragile or heavy, you may not want to ship it, so you may restrict to people who are close enough to pick it up. If you will only sell to people in Texas, don't distribute it to Australia. Check with your system administrator if you aren't familiar with the options you have for Distribution.
Different sites have different restriction codes available.
Distribution headers don't always restrict distribution, so it is still a good idea to include the target area in the Subject. Anyone buying or selling needs to understand that Usenet contains no mechanism to protect the buyer's money or the seller's property. This is posted to news.answers as well as other newsgroups. This is more efficient than posting the same message twice, as it stores less space on disks, takes less time to transmit, etc. However, there is rarely a reason to cross-post to many of the rec.audio.x newsgroups.