I've noticed that in many of the sales threads, posts containing minis with OS X preinstalled or SSDs with OS X on them may be deleted. However, there is an entire section of the forum dedicated to installing OS X on minis. This seems rather hypocritical.
I don't really think it's the forum's job to police who installs what on their machine, or who sells hardware preinstalled with software, EULA violating or not. Think of it this way: There is a question as to whether or not Apple's EULA is even legal, and if it is anticompetitive. By taking a stand against selling non-apple hardware with OS X on it, you may be supporting their monopolistic and potentially illegal agreement terms. I feel this is far worse than simply allowing users to police themselves. If Apple feels that someone is violating their EULA, let them take it to court.
In addition, by claiming you will delete posts that contain possible EULA violations, you may now be obligated to do so, and there may be trouble if you fail to do so. Have there been any other forums sued by apple because of sales of EULA violating machines by users? Have their been any lawsuits by companies because they claimed that a venue said they would police their users and then failed to do so?
On the other side of the issue, Craigslist was just sued because they supposedly catered to illegal activity (in that case, prostitution). If selling items violating Apple's EULA is against forum rules, shouldn't how-tos and support for OS X on a mini 9 be as well?
Just my two cents. I am not a lawyer, so I can't guaruntee this either way, but I'd be reluctant to claim that I would monitor and police what other people were selling on my forum.