Do not let this guy give you that "AS IS" B.S. The fact is that it is NOT an amiga high density drive, and he claimed it was. The AS-IS part of the deal is irrelevant. He stated that this drive was a High Density Amiga Floppy, and it is not. He lied whether it was intentional or not.
AS-IS refers to the condition, and you're not disputing that part.
Tell him you want your money back, or you are reporting him to both ebay and paypal. Now is a good time to practice being assertive. The worst that will happen is nothing... the best that will happen is you'll get your money back. You really can't loose by being a jerk at this point. Do your best to be one but be nice first. If that fails, go after him and make it a hobby for a few weeks.
-tom