Ah, looks like the registrars were still coming online when I tried. When I first tried Gandi, it didn't seem to like .app, but now it does and lets me choose "landrush," which I assume is the way to buy a domain during the EAP.

the actual two hard problems are cache invalidation and naming things, but cash invalidation doesn't quite fit into that :-P

It's only for the current early access period, so if you don't have a named project already I'd suggest just waiting for the period to end then your pricing is steady. We know how to design good auctions but I'm not sure there's any design that'll be kind to purchasers who can't easily evaluate the value of the item to them while still meeting other more basic goals.