[W]hat do I do with the old ipf6400? How does one go about selling a malfunctioning, hard to transport, specialty item like that?
I think you have described, in a nutshell, why the used printer marketplace is so limited, compared to (say) the used lens marketplace. If one has to go out and buy an inkset (or most of an inkset) to test / try to get working an older printer, then often it only makes sense to buy a used printer if the
total cost of printer plus delivery is extremely low / close to zero. Delivery cost (and/or difficulty) is usually substantial. And if the prospective seller will receive close to zero for it, why go to the trouble of selling it?
Example on a smaller scale, even assuming a working (or more often,
thought to be working, or working
last time it was used two years ago) printer: a new Canon Pro-10 with a full inkset is $450 with free delivery and some paper* from B&H, and recently I've seen them on eBay, from reputable sellers, new in unopened box but with the UPC removed for the rebate, as cheap as $290 with free delivery. You might think that the older Pro 9500 Mark II is almost as good, and worth (say) two-thirds as much. But almost any used Pro 9500 Mark II is going to need a new inkset, and a new Canon inkset is about $135. So if we deduct out $25 for the $50+ of paper than comes with the Pro-10, two-thirds of $425 is $283. But then with that $283 I need to use $135 on ink, leaving $148. Shipping and handling probably averages $50, leaving $98 for the printer. Is a seller with a known-working printer for which he might have paid $800 going to be too motivated to sell it when he can only net $98--and have to deal with the hassle of packing and shipping and the risk that the buyer will have a problem / won't be happy?
Yes, on eBay and probably elsewhere, occasionally you see someone buy a used Pro 9500 Mark II--needing ink--for $200 plus shipping, or something like that. All I can figure is that those buyers are clueless, or else heavily stockpiled in ink for that particular model. And there are lots of listings for this model in the $500 to $1200 range, but I've never seen one sell for that sort of price. That whole market appears to have little rationality.
*The Pro-100 and I think Pro-10 have these perpetual package-and-rebate deals where you get 50 sheets of 13 x 19 inch Canon either Luster or Semigloss paper, which retail for $50, plus 10 sheets of letter-size Canon Luster paper.