My philosophy has always been the seller has to tell me what they think a fair price is. I either agree or say no, I believe X is fair. But he has to start.
That unit right there was bought in 2002 with the 3 position controller and I wanna say 3 40lb bags of Traeger Pellets and delivered to Pasadena, CA for $850 ($1200 in 2018 $). The digital controller was put on in late June of 2003 for $275 ($375 in 2018 $) [yeah, they saw us coming back then] and it was sold in 2007 for $600 ($750 in 2018 $). Right after that new paint job!