I'm afraid I don't see this working all that well for a few reasons, including the point made by ScottJ about coverage.
Among his varied business interests, my DH has a small plumbing business, and new work invariably comes by referral without the need for advertising (though Yell get quite shirty when you tell them no thanks!)
Quoting for plumbing jobs without seeing them, no matter what the customer describes, is a no no as there are too many variables, especially variables which lurk under floorboards (lead pipes being just one

). I assume it's much the same for other trades.
The Boxby model is sound as it is basically about the movement of packages of defined dimensions between specified locations. However, the diverse range of jobs in various trades, including those which may require dovetailing with other trades, means there is an infinite number of unknowns, all of which does not lend itself to online quoting - except perhaps by cowboys.
I may of course be entirely wrong and this could be the Next Big Thing, but it's not something DH would consider using before the Forfar Dispatch or the Brechin Advertiser if work ever dried up - something that's most unlikely in the plumbing game.