I don't know if you've noticed, but everything on eBay is "rare"!
Seriously, rarity does affect value, but not by itself. It's rarity AND desirability that makes something valuable. And, yes, there are fewer of these than Bugeyes, but the production total for 948 Mk IIs is 3596 cars, and the other flavors of Sprites run around this number or even less. (According to "Sprites and Midgets" by Anders Clausager.)
As for price, of course it is set by what people are willing to pay, but not by what one person is willing to pay. The value is an approximate average of a lot of transactions, and by that standard, probably $4K-$5K seems reasonable from what I've seen. I was just wondering if there was something I missed.