Actually, the problem of devising this sort of a market (buyers appear at various times and don't want to wait, supply is unlimited) is pretty interesting and not impossible. The field of online mechanism design (where online means participants show up at various times) is quite popular these days, although I can't say I know too much about it...
However, it took about twenty seconds to see that there's already a paper that discusses a market virtually identical to this one (in theoretical terms). See: Incentive-Compatible Online Auctions for Digital Goods (2002), Ziv Bar-Yossef, Kirsten Hildrum, Felix Wu .

Their analysis: We first show that no deterministic online auction is competitive relative to the optimal fixed price revenue.
Hmm... doesn't sound like Apple needs to run out and implement an online auction market for itunes quite yet...