When say revenue do you mean Gross Merchandise Volume?
Because that would really be a small market if you stuck to SF. I've used infousa.com to get an idea of number of businesses in an SIC code. To help me get an idea of the size of a market.
Sorry, I see it is revenue. $25-$75 per business. The problem to me is that puts you in difficult range at the unit level. Too cheap to sell in person or over the phone. So you would need to use some kind of self service system. And the concentration of users probably isn't dense enough to get word of mouth and/or viral effect.