This seems to be prevalent when someone offers either a system or games for sale in the videogame forum. Is there any rule against this? I don't seem to find it often in the DVD Exchange forum when people offer old catalog titles for new releases.

Mean-spirited thread crapping is frowned upon. The best thing for you to do is to use the "report this post to a moderator" function when you see it and let the mods responsible for that forum judge the matter on a case-by-case basis.

Who would crap in an exchange forum? That's just stupid.