I’ve never sold a screenplay treatment or pitch so I may not be the best person to ask about this. While I know this does happen it’s usually established writers who make these sorts of sales because the only people buying pitches and treatments are established producers and studio executives – and they’ll usually only meet with established writers.
Think about this from the producer’s standpoint. When a producer buys a pitch or treatment they’re taking a great risk which is something they’re very unlikely to do with a beginning writer. If they buy a pitch from a writer they’ve still got to get it turned into a good script, which is really hard. At least if they buy a pitch or treatment from an established writer they already know he can write so there’s at least a reasonable chance that he can turn his idea into a well written script. This is not a risk that most producers would take on an unknown writer. And if I were a producer I wouldn’t either. And neither would you.
Beginning writers tend to overvalue their ideas. They sometimes think that they have brilliant ideas when they usually don’t never do. A good screenplay needs good ideas, or course, but even more important than a few good ideas is good execution. Coming up with a few good idea isn’t the hard part, it’s weaving them all together into a great script that’s tough. Producers understand this. So if you’re a new writer do yourself a favor and spend the time writing the script. If you want to write a treatment first, then go ahead, but then turn that treatment into a full screenplay.
You should read this post, too, Should I send a query letter to an agent or producer for a script that I haven’t written yet?, because it covers some of the same ground.