Okay, if you're eBaying, watch out for the differences between the North American and European editions of the Game System, Kellar's Keep and Return of the Witch Lord. I would assume you want the NA editions because of the extra loot they come with... and more Body Points for Monsters. Several of the quests have been tweaked in the NA editions as well.
For the Game System, the easiest ways to make a quick distinction is where it's coming from (UK, likely EU edition, US likely NA edition). Look for the Armoury Guide in a picture, the EU edition doesn't come with it. It's quite large as it spans about 3/4 of the box length:

As for the expansions KK and RotWL, they are easily identifiable by their boxes; NA on the Left, EU on the right:


The North American expansions come with new artifact cards to cut out from the back of the book cover (often already cut in auctions) and the Alchemist's Shop on the reverse of the books cover.
You can still find sealed KK and RotWL NA expansions on eBay monthly. They usually go for $75-$100, but sometimes (like recently) they can go for $40-50. Used ones tend to range around $50-60 at the moment but sometimes you can find them cheaper. I picked up another RotWL last year in great condition (used, but no wear on the box cover at all) for $20. These are all plus shipping by the way.