The real mystery to me is the stuff going for decent prices at auction and the same stuff just sitting on the bay - no takers. I've bought from Stout, Trainz, Cabin Fever, the bay and this forum, as well as new stuff from brick and mortar hobby shops - most being forum sponsors. I decided what I was willing to pay and passed when it got out of hand IMO.
Auctions are simply what two buyers at the time are willing to pay for an item. I was recently bidding on a NIGHTHAWKS SCENE KIT against one other bidder. The hammer price including fees ended close to what the retail price was. Turns out there are a few kits still available from the original manufacturer, so it was not a "rare" as I had thought. Won flex track and several DownTown Deco buildings, so overall a good shopping day.
Looking back through my inventory sheet, prices are up 2x to 3x especially anything MTH from 2 years ago. Common Lionel cars - hard to sell at any price. Like anything - it depends on what people are looking for.