Even if you think you'll probably do the job yourself, it's good preparation to seek estimates from professional home painters, whether your painting the exterior or interior of your home. Then you'll have a financial point of comparison and you may benefit from what a home painting contractor has to say about the condition of your home, color choices and types of paint available. Let the painter make his pitch for a professional job before you decide what to do. You can still opt to do it yourself while having learned something worthwhile.
I used to work at Sherwin Williams. The best thing to do is specify that you will purchase your own paint. If the painter objects strenuously, he was plaanning on making money in the ways this article outlines. You might pay a few dollars more for paint, but you will have control of the quality of what goes on your walls. Never skimp on paint quality.