Here is a simpler example for a Uber-like implementation:
* Taxis would be advertising their current position and whenever they they are free or not with their phone.
* Potential customers would advertise their current location and where they want to go with their phone.
* Broker services would match taxis with customers
Obviously it's more complicated than that, there could be a reputation service associated to an account for example and your own algorithm running on your phone that chooses the combination of trust and pricing that you want.
If the developer had it's own service (like his personal website) that hold the list of services that he provides, his availability and general price. Then brokers could connect to this and match it with customer requests. Same on the other side, a customer should be able to submit his project proposal and let the various brokers pick it up and match the appropriate developers.