Well I've done it and it has resulted in people getting a good coop at a great price or free. I think if our right to post links to used coops is taken away, then the right to post links to new coops would have to be forbidden as well. The coop design section, as I understand it, is a place to share ideas and information with each other. People go from not knowing where to start to having a great coop because of information provided by members in the Coop design forum. This a a great community of chicken-loving people and one member can't expect all posts to be relevant or of interest to him or her. I know they aren't all of interest to me! Could just skip those posts if you aren't interested or it isn't in your locale?
Just an idea.
Mary