I've always done it. Sell the current unit before AppleCare runs out and you get a good price for your tower/whatever.
But when I sell my iPhones...it's nuts.
I paid $300.00 for my 3Gs and $40? for AppleCare last May off of Ebay.
I got an iP4 for anniversary/birthday gift so I am selling the 3Gs.

It's almost busted $400.00...

Making a profit on a used electronic item...lol.