Around here used
chipper prices are crazy. That gives me hope, as I have been thinking of selling my Wallenstein BX-42 (4" x 10" throat) and moving up a size. If I go by pricing on the internet, I can almost get back the original cost of the Wallenstein from 2012 when I bought it. Of course new ones cost quite a bit more now in 2019, but just looking at the long term cost/return, it would be amazing to sell a piece of equipment for near/at what I paid years ago.
BTW, gravity feed has worked fine for me, and the machine sucks in material extremely fast. I learned to drop material in and get out of the way.
The downfall of a gravity-feed
chipper is that it doesn't do well on leafy/twiggy material unless that material is being dragged in with a thicker limb (either attached to the limb or fed at the same time). If I have a lot of leafy/twiggy material, I have to alternate that with bigger limbs so the bigger limbs can periodically clear the feed chute by dragging everything else through. That's the only downside I have found, and don't consider it to be a real issue for my occasional use (few days every couple months). If I had more frequent use I'd probably want hydraulic feed.
If looking to buy used, I'd say keep your eyes and ears open and wait for a good opportunity. Be flexible. And beware of the cheapo Chinese knockoffs. They seem to give mixed results when brand new and I imagine used units are a crap shoot.