Well it depends somewhat on what all comes with it. Does it have a spool of wire on it and how much left? What about gun consumables? Is the gas cylinder full, half full or nearly empty? What size cylinder?
When I bought mine a couple of years ago, I paid somewhere between $1800-1900.00. That was brand new. So depending on how bad you want to sell it, and what the market in your area is like, I would say between $1500-1700.00