The CD400 works and is in fantastic shape, and the seller included 3 batteries and an unopened pack of 5 Memorex Mini CD-RWs! Fantastic deal, I'll probably make a quick video later today showing it off.
The Zip 250 drive on the other hand... It looks to be in great physical condition, but it's more dead than I've ever seen any other device be. It's supposed to draw its power over USB, making it more convenient and portable by ditching the wall wart. But when I plug it in, absolutely nothing. Not a buzz, not a flicker, Windows doesn't see anything, nada. Insert a disk, well, now you have to use a paperclip to get it back out. I mean, absolutely bricked.
And I have no clue why! Maybe the original drive shipped with a "special" USB cable that had some specific wiring, and won't work with any others? But that would be immensely stupid, so that can't be it. Maybe some power supply cap has gone, and if it's replaced then this'll work just fine? Too bad I don't have a soldering station or any experience in that field...
Ah well, the seller graciously accepts returns, so I guess I'll be sending this back and putting the refund money towards another spin on the eBay roulette board.