I suppose I'm after a few basics - not the whole line - on eBay. Some of my players have played a little in DS (I have not). I don't think I'm after a full campaign, I have enough plans already, but perhaps an extended sojourn in my upcoming Planescape campaign.
OK, if you want just the basics:
Avoid the "Racial Supplements" (Elves of Athas, Thri-Kreen of Athas, etc.), since you already have etablished PC's.
Perhaps focus more on the 'Setting" itself.
For that, try:
"Valey of Dust and Fire"
"The Ivory Triangle"
"City-State of Tyr"
"City by the Silt Sea" (Boxed set, and probably my favorite piece of the line!!)
Monstrous Compendiums
Also useful?
"Dragon Kings". (Check out the high level Psionic Enchatments!!)
"Defiler and Preservers" might be useful, if you toss a powerful Defiler at the party! (And some NASTY new spells-- but remember, this was PRE BoVD!!)
Also of use might be "Psionic Artifacts of Athas". Somewhat of an "Exotic" feel might mesh well with Planescape, and some cool unique artifcts. (Check out the "Obisidian Man of Urik" in particular.)
For adventures?
As I said they vary in quality, but try "Black Flames", "Dragons Crown", or "Forest Maker" (The last is useful if you want to explore the metamorphasis of Dragons and Avangions.)
(And be sure to check out the "Cosmology" section of the Core Book, if you're trying to mesh it with Planescape!!!)