I've already seen the answer for "Where to buy" questions and http://blog.stackoverflow.com/2010/11/qa-is-hard-lets-go-shopping/.
They both say we shouldn't allow those questions. According to the blog, it is also in the Super Users FAQ.
Does the same apply to "where should I sell" questions? The situation isn't identical, and isn't necessarily always as subjective.
Here's such a question: