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.
His team is very meticulous, very careful, very clean, and very courteous! They're also very accommodating. For instance, I mentioned a couple spots under some angled siding where I thought a little extra filling was needed, and they obliged without question. Same thing with a request for a stain-proof primer under a balcony. Doing these kinds of little things without question or extra charges is really nice!