Keep in mind you can get a base model 2011 Mac mini for 300 - 400 on Craigslist.
350 is really top of the mark for a 2009 mini. The thing with minis is that they devalue primarily because newer and better minis come out and are for sale at comparable prices. The older machine necessarily drops in value where newer and better tech would otherwise cost the same.
Complete nonsense. A quick look at eBay shows the OPs machine selling average $450-500.