3DPrinting

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.)

Give it to someone who wants it and ask money in return.

Country/location and printers?

e.g. In Germany facebook is dead but from what I hear it is still used in the USA. Smiliar a Ender 3 has a diffrent target audiance than a Raise3D.

Where are you from and which printer are you selling and for how much?

I would say that it realy depends on where you live, for Germany i would recommend eBay Kleinanzeigen.

For ebay you always want to take pictures first, package item properly with reinforcement/packaging material, weigh and measure the package. The shipping estimates are based on these metrics, so you need to do them ahead of time.

Yep FB is the way to go for local sales. I'm in a large metro area and exclusively use it.

I posted on Facebook marketplace and it was sold within a day. I live in a town with 3,000 people in it. Facebook is the internet to most people here. Try there.