I thought things were settling down on eBay but I have witnessed the same as you mike recently. I really think that there is just a small group of people out there willing to bid crazy money on this stuff. The worst I ever saw was over 300 bucks for a box of silk span. I saw a seller offering some banshee kit parts, enough for 2 kits I think, and they went for well north of 300 bucks also. I don't remember if there were plans or not included. Like the old saying goes, it only takes two participants at an auction that really want the item to drive things crazy and I really believe it is just a very few people in the whole country driving these prices. I usually check the bidding record and you can see what the activity is and who is bidding.
Some of it is genuine need, but don't underestimate people wanting to buy credibility. Effectively, they are trying to make it so they can claim to be "old school"; that is, they were doing it "before it became cool".
I saw the same thing in rockets in the early days of eBay, the most notable being a "original" Astron Space Plane kit (with the green corrugated hang tag) for $1600, and another one where someone wanted to buy just the *box* for the Centuri Little Joe II kit for $400. I know the purchasers of both of those, and they were desperate to make themselves seem like big shots by having a bunch of old stuff around, the implication being that they had been doing this so much longer than you and I that they were super-cool.
BTW, an Astron Space Plane kit sold for $1.80 and consists of a very crude body tube (wound from manila file folders), a balsa nose cone about 3/4x2", and a couple of 9" sheets of the worst 1/16 balsa you ever saw, Sterling would have rejected it. The only exotic part were two 2-56 x3/8 nylon pan-head screws that even now sell for about a dollar for 1000 of them. The heavy wood was actually necessary, because if you built this model too light, it was either unstable on boost, or did death dives on the glide.
Also, don't overlook the possibility that someone is trying to buy it up to flip it for 2x, one $10 sheet at a time. I think that is about what an entire roll of Polyspan costs!
Brett