Beats the piss outta me It could be as simple as a community joining together and figuring out how they'll go about it, to an automated system, perhaps a website, which collects funds to be distributed by a trusted member of society (LOL) for the contractors who auctioned for the job.
What about a two-way auction; workers decide on prices they'd like to work for, and people decide how much they're willing to spend for the projects--people as in, entities, groups of people, or maybe one charitable rich individual (LOL)
Two way auctions is interesting system. Still, would need some sort of common platform. First get group of people agreeing on what will be bid on at on what terms including placing a seed bid in, then people can bid into their part into system and at same time other side provides bids to provide the service. At some point these two would be met and possibly after certain time the lowest bid for providing service would win and the bids for service would be moved to trusted party possibly.
Not really good explanation, but I think this kind of system might be needed at some point. It's not perfect as there needs to be one or two third parties or trust in service providers.
Still, the question remains is this efficient use of resources, mainly would people bidding to get service use their time to produce something. While some party could be much more efficient in using taxed resources.