Something which wasn't mentioned here is that many retailers don't want you to know - they want you to come in to the store. If all of the retailers have you come in to the store, there's an effective block on this sort of thing working.

Also, many retailers have "we'll honor competing prices on things but only if they're in stock" policies. This would make it too easy for people to play retailer X off retailer Y.

I noticed on milo.com that most of the items I looked for "might" be in stock. Well, of course they might be - I still need to call/drive.

A different approach would be a push - let me push out what I'm looking for, and have competing retailers contact me with replies. Have the service cost a bit for retailers to use - not a lot, but something to prevent rampant spamming. Build in location, so retailers are only getting requests from people in the area, and have retailers use the system on a regular basis. Just like someone should check voicemail every few minutes (30?) someone could check serviceX, and reply if they have the item in stock with the price, terms, etc. "Click here to hold the item from company Y for 2 days" etc.