No, I meant a legitimate intermediary. If builders get to overcommit and then cancel on their customers, why shouldn't a customer be able to do it too? The intermediary would just offer a service of doing that for you - booking multiple companies for your construction project, waiting until the last second, and cancelling all but one.
Why couldn't they? I'd imagine the only problem is it is an ineffectual strategy for a client. There is a lot of money involved, people are already doing everything they can think of to get the lowest price.
It is absolutely a dick move to lead someone on, but in the grand scheme of things a minor problem. Construction is a mess of problems.
The developers aren't held to a timeline in the contract. They just stall forever, working on the ones willing to pay the most. If they ever ran out of better customers, they would eventually get around to the cheaper customers.
They've done something unethical, but they haven't broken the contract.