I just noticed you asked how to do the on-line trade in...I don't think Tirerack for example deals in the used ones...but if you are having them shipped to an installer for example...you might talk to them. E Bay, etc...can let you sell them...or you can run an ad in the paper, stop by a local tire place, etc.
I go through a lot of rigs...and I get tires from a local dealer and from tirerack for example...and I used to also use my dad-in-law's gas station...and do my own mounting, etc...what ever was convenient or cheaper depending upon the circumstances.
Places that fix flats usually have a rack of used tires that people with sliced sidewalls/slicks due for state inspections...etc...come to for a quick fix of rubber....they will buy the used tire, and sell it to these people...you can even drive over with the tires on the truck, mention you are getting new ones, and see what they'd give you for them...
After getting the new ones...drive the used's over to the place of choice to get paid.
Any help?