Recently an item was advertised for sale on this forum.
The poster probably had not read the rules at the top of the page.
Here is the rule governing items for sale.
8) This forum may not be used for the sale of items of any type. It is not for advertising other than announcements of pending railroad related activities that would be of general interest to the people who frequent this forum.
Links to sites where items are for sale are not usually considered to be advertising items for sale. Overuse would change that consieration.
The best way to post an item for sale would be to advertise it on railswap. RyPN has now set up a classified ad section too.
If advertised on a site that allows advertising, then one can post a link on this site to the site where the item is advertised. This meets the rule. I will add that the item linked must be somehow identifiable as railroad related. It can be railroad art, clothing, even kids pyjamas with locomotives printed but it must be railroad.
A classified ad section was considered during the forming of the new Chaski site but the idea was dropped. It may be considered again in the future.
There is too much potential for it to become a free for all. A number of additional volunteer people would have to be found to manage the additional work load of managing the classified section.