Are router questions good questions for NANOG?
If your question is "how do I do <foo> on a <bar> router?", this question is best asked on a router-specific mailing list. If your question is "How do I get <vendor A> and <vendor B>'s implementations to work together?" then that is useful for NANOG.
Where's a good place to buy a used <foo> router?
This would be a vendor question and off-topic for NANOG. Your best bet is eBay, but beware of vendor maintenance policies related to the used equipment market. This is not meant to discourage you, it's to tell you to be an informed consumer and know what you are doing when you buy on the secondary market.