I have a brand new Rock river predator upper Wylde Chamber, with an LRB Lower, Timney Trigger and a Millet $400 scope for $900, and a bipod. Never fired. Selling for a friend. He built it and put it away and changed his mind. This is a $900-1200 gun plus $200 for the trigger I have a similar gun, same upper and it shoots 1/4" groups.
The thing is, why would somebody buy something used or secondhand if they can get it fort the same price new? You have to take a loss. It depends on your tolerance. Your gun might be worth $750 to someone who really likes you.