I try to buy directly from the sawyers, primarily because it's cheaper. I even bought some green lumber once and dried it myself. If you have the space, equipment and time, buying lumber rough gives you more back for the buck, and more flexibility. With rough lumber, I have the option to cut a 2' piece of that 8' board, negating the twist and treating it as 5/4 stock I can mill to 1" thick. If it was S2S bought at a store, that board would just be 4/4.
I've also bought lumber from Klingspor and Capitol City lumber. Klingspor has pretty reasonable prices, and it's already surfaced on 2 sides (S2S). When you buy rough lumber, you will inevitable have boards with checks, cracks, twists and knots, and those flaws aren't always visible until milled. So there's quite a bit of waste. When you factor that in, the higher price you pay for lumber at Klingspor is a pretty modest markup. Also, buying lumber from sawyers is generally opportunistic. Scott Smith has a reliable source of oak, but with most sawyers, you pretty much have to jump on something when it becomes available. At Klingspor, I can get what I want 99% of the time, in a comfortable heated/ air-conditioned store. That's worth something too.
So how can a sawyer do a better job of selling lumber? Information is key. Advertise what you have, with prices. The only cost is the time it takes to type the stuff in. Accurate grading helps too. For those with a lot of space, you can buy lumber in bulk. So when building a bed that requires long boards, you pick out the ones without defects and save the shorter ones with knots for the entertainment center you're building next week. But if you're a small time consumer, you can't afford that, you have to be more precise about what you buy. Not everyone has a truck/ trailer, so the ability to trim boards would be a big plus as well.
Gee, I guess this is why we have retail