Marko
That's a 3 ton unit, good for a small house, 1400 to 1600 s.f. I'd say 300 dollars per ton, at the very maximum, probably less. There are a lot of costs for a licensed tech to remove it, store it, sell it, install it. A homeowner would have to have one to do the work, so he'd pay for the unit and the tech for about two thousand total. That assumes it doesn't get damaged, doesn't currently leak. It is a risky buy for someone, so the cost must be low enough.

Boe
If it works why get rid of it it not that old. But to answer your question, $500 max If someone offers you 400 ish Id take it an run. There is no "guide line" for use equipment its what every what someone will pay. If you want a good place to refereance of all places check craigslist and see what 3 ton AC units are going for. Yours is not that old and still may be under warraty (the outside unit) if you bought the extended 10 year warranty. Like I said its not that old so why dump it. But thats your call.