I can see some questions like 'where can i get the cheapest X' and other members might have X and want to sell.
I have mixed feelings as to whether or not that should be allowed, but it would be interesting to see some opinions on it.
I don't like the idea of SuperUser being your personal Amazon or eBay. The questions asking "where I can get the cheapest X" are already annoying as it is.
Edit: I should probably revise this some now that I am actually a moderator of the website.
So far there have been lots and lots of questions looking for product recommendations when it comes to specific issues ("I need a free/cheap video editing software"). These questions I have no real problem with (as long as they are not duplicates of something already posted). This will allow for people to post their recommendations and there can actually be a correct answer (what the OP decides to end up going with).
I find these type of questions are much better than "Where can I find the cheapest router" or "I need to find a place where I can buy [X] for less than $50". There are plenty of websites out there already which allow you to do specific product searches and compare prices among other websites, we don't need SuperUser to devolve into that. I do think that SuperUser's community can be a helpful source of product recommendations, however.
Ugh, let's not become Craigslist without the "adult services" section. I can't imagine anything worse.
There was a question where someone was asking if he could still get data off of some old 5 1/4 disks. All I did was append to my answer that I had some spare parts, and it got down-voted immediately. I have since removed that from my answer.
I wasn't really trying to sell him, or anybody, anything. I was just trying to be helpful.
All I saw today was somebody asking where they can buy hardware for cheap...I suggested newegg, and that was it :)
If someone's asked for the cheapest X then I can't see there being a serious problem if a vendor of X comes along and says get it here. There could be issues around the wording etc, but it would have to be on a case-by-case basis.
The problem might be that the question's been posted in the first place.
Where there would be a problem is if the vendor posted the advert (for want of a better word) as a question.
Don't forget you can flag as Spam. The worse case is "How do I solve X?" with an answer "Use product Y" (where Y is a referral link, and doesn't actually solve the problem).
It would be neat if the system could automagically either de-referrer them (or potentially better for the site as a whole) automagically substitute for a StackOverflow referral tag. For things like books,
I also like Wikipedia's approach if sending you to a landing page, with referral links for all the major online bookstores (based on ISBN)