I have experience buying, not selling, but should still be able to help.
Local sales are the way to go for 3d printers. Local sales also allow people to see the printer and check that it works as they expect before committing to buying it. Most people will pay more for a known-working printer than a the-owner-says-it-works one. Local sales also avoid shipping which, in addition to the downsides that you've discovered, caries some risk for the printer. The downside of local sales is that you might have to wait for a long time until someone nearby wants the type of printer that you have (or until someone who doesn't live nearby opportunistically picks up the printer as they were going to pass near you anyways).
Nealy everywhere has a Craigslist-like for local sales, which could be Kijiji, Letgo, FB marketplace, etc. There's no harm in listing the same printers on multiple sites to see which one gathers the most interest. (Just please make sure to take all of the listings down once the printers are sold.)
Others have suggested greasing the dowel or using bearings, and if the issue was friction, then they'd be right.
If the issue is inertia, then this won't help. Accelerating a given mass to a given velocity requires a certain amount of energy, no matter what.
What could help is something similar to a Huygen drive, where the filament is looped around a large wheel (large enough that the filament can curve around it without breaking) which is able to move against a spring or counterweight. This would allow the movement of the spool to be "averaged out" rather than accelerating and decelerating sharply on every extrusion and retraction.