The software on them is not licensed for production use. So, while it may be functional, if you ever have a problem, you're in a black hole as far as Cisco is concerned. As I understand it, the license for production use of Cisco IOS, CATOS, and the PIX software does not transfer if used equipment is sold. That'd be why most lower-end Cisco equipment on eBay generally is geared toward lab setups.
I could be wrong, though...and if I am, someone with more knowledge of this, please correct me...I don't know the Cisco EULA as well as I should.