It's not just you. I, also, am finding the well known good nibs are being aggressively bought, driving up prices. The winner of the Spencerian #1's you mention stacked up a couple of bids higher than $531, which indicates he would have paid a higher price if needed to win. Part of it is individuals buying big lots of these nibs and then reselling them one or a few at a time at higher prices, sometimes much higher prices.
I'm all for capitalism, and for people making a business out of nib sales, but it does make it more expensive for those of us actually using the nibs. It certainly has priced me out of Gillott 604ef's at $25-30 per nib. Millions of these were made, and even a hundred years later, there are a bunch of them out there. I'll continue the search, but eBay is not the place to find them at a reasonable price anymore.