Start with an online store, (such as Amazon), which allows individuals to offer items for sale alongside the store itself, and other established vendors.
(I.e. products have a generally recognised base price but occasionally turn up much cheaper when items are sold off by close-out vendors, or by
individuals (like a high-tech garage sale...))
Customers would submit a request to be put in a queue for whatever item they were interested in buying; they would specify a price they're willing to pay, and a minimum reputation level of the seller.
When the item becomes available at or below the price they specify, from a vendor of an appropriate level of trust, they receive an instant SMS telling them the item is available. They have <X> minutes in which to buy the item before the next person in line is notified.
If they want a longer period to make up their minds, they can buy extra minutes when they submit their initial request. The payments for extra minutes are deducted by the store before the offer is transmitted, and if payment fails, the offer does not go out to that person at all.
Some variations on the theme: 1) if the price you wanted to be alerted on is higher than what the vendor was going to charge - tough, they'll send you an offer at your requested price; 2) you have to pay even for the shortest alert time, not just for extra minutes (this may be necessary anyway, to keep out deadbeats putting in multiple requests under different IDs); 3) you can pay extra to jump the line (the way some juke boxes work now); 4) price is like a bid, offer goes to the highest bid first, not the first in line;
This time-sensitive model brings the high-pressure sales and the excitement of a woot-off to everyday transactions.
Credits: borrows from the Amazon business model and the Woot Inc business model. Possibly to a lesser extent to the 'buy a bid' type of thinly disguised gambling site. Relies on the new trend for everyone to be connected 24x7.
Personally, I would use a service like this, but then I'm a Woot fanatic already...