Having been a pretty vigilant eBay watcher for the last month or so, I have concluded that I am probably not going to find what I am looking for there. There are bargains to be had, for sure... two types of bargains, as best as I can tell (specifically in terms of digital pianos, if not items in general).
The first is the imperfect item. if you are willing to take a keyboard with a key or two that doesn't work, or something with a crack in the cover, or perhaps a hissing speaker. And for those of you out there capable of fixing these things, good on ya! But me, I know my limits. Fixing things designed to handle electricity coursing through them at lethal levels... that's not one of my gifts.
The other is the big ticket item. People who, within the past couple of years, plunked down large amounts of money for Clavinovas or high-end Rolands, and either have given up, or upgraded. I have seen a number of these go for surprisingly little, given how much they cost new. Of course, I think a recent Lexus LS would be a good buy, too, but that doesn't mean I am about to replace my 2002 Ford Focus with one.
I'll keep a casual eye on eBay and Craigslist, just to make sure nothing compelling falls through the cracks. But I reckon there are hundreds of others doing exactly this. This means that, when things sell, they go for a generally fair price.
Upshot: might as well buy new.