It looks like the grill market is a seasonal thing and tied to the weather. In areas that don't have large seasonal weather changes (SoCal), it's more of a continuous process, although I think after watching the market for the past couple of years that we don't get the amount or variety of used gas grills seen in other parts of the US, and used charcoal grills seem to be the hot commodity in my area. Also, grill sales seem to slow down as the price approaches $200, so I think there are very few flippers in this area (could be wrong about that). One thing I do know is that, sooner or later, they will all sell at some point.
Many (or most) apartment buildings don't permit either charcoal or propane grills in my area.