If you want to buy a like new, but refurbished Mac that Apple warranties for a 1 and up to three years with paid AppleCare, then visit their refurbished page on their site.
http://store.apple.com/us/browse/home/specialdeals/mac
However Apple will charge sales tax of the state your living in because they maintain a store somewhere.
Now you could buy new online elsewhere and not pay sales tax provided the site your buying from doesn't have a physical location in your US state ( Amazon is in Washington State), also you get the 1 year Apple warranty as it's a new machine, and thus can buy AppleCare.
You can't buy AppleCare or get a warranty from Apple for a used machine, unless it's from Apple Refurbished.
If you qualify, you can opt to get a educational discount from Apple, but they still charge sales tax/
If your really scraping to get a Mac, then I suggest you put it off or buy a cheap PC until you can save up more money.
There are all sorts of other things you need with a Mac, a case, storage, software, gadgets etc., and so forth that also eats into your wallet.
Also unless your getting the MacBook Pro 2.2 Ghz or better models, the ones below that are really not much better than generic $600 Window PC's you get at the office store. Same cheap shiny screens and poor overheating integrated graphics.
Why pay $1500 for something you can get for half price that does the same thing and dies in the same amount of time?
If you need a powerful laptop and have to spend over $2000 to run heavy duty programs, then a MacBook Pro is worth it over a same powerful Windows PC.