You are correct -- once you put in a hard drive you don't need the DOC.
Stuff on EBay has been way over priced lately.
And yes, you CAN resell your DOC if so inclined. The 48MB DOC 2000 is a nice item, and since there probably several around now (because people are doing what you are doing -- putting in a hard drive) I would have expected the price to be a bit more reasonable. Seel em if you can! Take advantage of it, while you can...