How about adding something either to the rules or even another sticky entirely, that is more general sales advice rather than a rule per se, and point users to it before they create a new post in the forum. Something like make sure to do a quick check on ebay, newegg etc and mention that pricing at the average selling price (market value) or below will likely result in interested buyers, but selling at, near or higher than retail price from reputable vendors will likely result in little interest in the sale etc. Also, beef up the advise for buyers generally, but new buyers in particular, things like make sure you check what the current market value is for the item, as an average, make personal contact with the seller via PM, Email, Telephone etc and satisfy yourself that the seller is genuine. Do not hand over cash via wire transfer or other means that is untraceable after the fact, or enter into any deal or contract where you feel uncomfortable etc. Make sure buyers realize that if it does go sideways, they have done all they can to protect themselves from any financial pain, likewise the same goes for sellers, make sure they realise that buyers can be scam artists also and that they also should take appropriate measures to make sure their buyers are genuine and only accept payment via recognised safe platforms.
I'm not meaning that this should be an enforceable rule, just some friendly advise for sellers and especially new buyers. It is just common sense, but it's amazing how easily you can get carried away in a sale and not do things that would otherwise protect yourself if it went wrong.
I like the for sale threads as they are. Generally folks are well mannered and sales of goods take place trouble free for the most part, as far as I can tell from reading through the various posts anyway. It would be a shame to lock down folks to strict rules, after all, the majority of these posts are members that are simply flipping their unwanted/ unused equipment to get a bit of money towards their next project. Having folks ask questions about items that are for sale and having those questions answered is all part and parcel of trading goods, and like any post, it should reasonably stay on topic. If someone sets a price for their item, that does seem on the high side comparatively speaking, it is obviously their choice and will probably result in little if any sales. Likewise if someone else points that out to them, where they can show evidence of it, and as long as it's a polite nudge intended to help the seller, I see no harm in it. I would draw the line though at anyone trying to badger the seller into lowering their price, they have been advised of the situation and it's their for sale thread. As for moderation of said forum, obviously it's a balancing act. I appreciate that it must be hard for moderators to be able to be seen to do the right thing, without having at least some blow back from those being moderated or asked to behave, you can't please everyone all the time, but common sense and good manners should guide folks when posting, just like posting in any other forum and when the time comes where someone crosses that line, I'm happy to see moderators step in and counter that behaviour. The forums at STH are a "happy" place for most of the members here, I would like to see it continue that way and if that means folks sometimes need their adrenalin pump turned down a bit, so be it