So I've done a little more research now.
In the pbook world, the large publishers generally work through distributors, who then sell to the bookstores. Dealing directly with a ton of individual stores just isn't cost-effective for the publishers. So it's no surprise to find that they're using the same approach in the e-book world.
From what I've found, if you want to set up an e-book store that sells DRMed books from the major publishers (assuming you don't have the size, money, device, and unique format of a Sony) you sign up with any or all of the following list of e-book content distributors: MobiPocket, LightningSource, or Overdrive.
MobiPocket we all know. They distribute books in the MobiPocket format and also operate their own retail site. Anyone selling DRMed MobiPocket books is likely buying them wholesale from MobiPocket.
Lightning Source is part of Ingram Digital Group, which is part of Ingram, the largest pbook distributor in the US. Lightning Source also sells POD services, by the way, as well as e-book conversion services. Lightning Source distributes e-books in Adobe, MS Reader, and eReader format. So any e-booksellers selling DRMed books in those formats from major publishers is most likely buying them wholesale through Lightning Source. Here's their FAQ for booksellers:
https://www.lightningsource.com/FAQbookseller.htm
Overdrive is the company providing the services for many of the public library e-content lending systems. I couldn't make much sense out of their offerings for resellers, but one of there services is to provide an entire
hosted, brandable store.
So, let's say S&S drops the list price of a title. That price change has to go to Lightning Source, MobiPocket, and Sony. MobiPocket and Sony have to update their price lists. Lightning Source and MobiPocket have to pass the new price on to their dealer networks who each have to update
their price lists. There's all kinds of room for delays and drags in that process.
Anyone for starting an e-bookseller site?