Generally the going price for a used pit in good condition is roughly 50%.. However, this in not only a high end unit with little use, but a USA model. These are excellent pits that have an excellent reputation..
Depending on the urgency to sell and your willingness to hold firm, I would opt for something in the 60% range.. Time is on your side with Summer just starting...
Just make sure the pit is spotless and ready to fire up. Any pellets might sweeten the deal if you have some you aren't going to use..