I have the best luck with FB Marketplace: It gets the best reach of users, and you don't get near the spam or flakes like you do on CraigsList. You could also try apps like OfferUp, although I don't have very good luck there.
If you live in a larger metro area I'd try and sell locally. I stick to local sales since eBay charges a lot, shipping can be hard to estimate, and big boxes get expensive to ship really quickly.
I've not experienced it myself, but I've been told that eBay promotes listings with "free shipping" so you may be better off baking the shipping fee into the item cost itself rather than charging shipping separately. I'm pretty sure eBay takes their fee out of the Shipping cost too, so it doesn't matter "what" the money is for, they take their cut regardless.
P.S. - I've experienced similar brand trends as Les. Rainbows command the highest resale, by far. Even current gen Kirbys are hard to sell above $200. Kirbys can be found EVERYWHERE for CHEAP and people don't see any difference between an Avalir 2, 1, the Sentrias, or even older generations. Unless they care what color the bag is, why pay more?
P.S.S - Take lots of pictures and have everything be as CLEAN as possible. I even post pics of the brushroll, inside the bag chamber, etc. People don't want to buy someone else's dirt.
