The main challenge would be to convince business owners to publish data about their stock to the web. The rest should be easy.
The absolute only idea that I had which might work is to provide businesses with 100% free automatic (rfid or whatever based) inventory systems of whatever size they need. Yes, that is a large expense.
However, in exchange for the free inventory system, you get access to their data and the ability to post it online. You give people the ability (but don't force them to) buy the item from their mobile phone/computer quickly and easily, so when they show up they have a zero balance and are ready to go. Then you take a 2-5% cut of that (just a hair more than credit card transactions) for the online business referral.
TL;DR- Give business owners something expensive and massively valuable in return for their data, and the ability to let consumers buy their items through your referral system online.
Unless the time and money spent on the effort is going to pay off, and pay off quick, a store isn't going to bother.
The major work wouldn't be in manpower, but in hooking up the db to something that can index it.
For large companies, it is a multi-million dollar investment to build a warehouse management system capable of showing near real-time levels of inventory. Besides the capital costs, companies can have very specific workflows for managing inventory levels. Also things like drop-ship vendors and sourcing items from multiple sources can make it extremely difficult to calculate or predict inventory levels prior to a sale.
There's a reason Milo is starting with big-box and national retailers.
1. The difficulty and expense associated with calling and converting smaller retailers
2. The amount of effort required by smaller retailers to integrate with your system coupled with the fact that smaller retailers are technologically lazy and don't readily understand the benefits of integrating
The "easiest" way might be to replace their inventory systems yourself, for example. This is more-or-less what OpenTable did.
We're working on tagging our products with RFID. That way, we can show the inventory of the products online for consumers to see. Not only that, we can help our retail partners keep our product in stock.
It's at no cost to the retailer and relatively simple.
Walmart has been trying to get a similar system implemented for years (they're currently testing their latest iteration on denim - all denim is tagged supposedly).
We do not sell to Walmart, we're in a very different channel.