Where I am there are several places that buy logs or wood close by. Besides the saw mill there are three log yards. They just buy and sell like a broker. Both the mills and the yards have a spec sheet which is just a list of what they are looking for and how much they will pay for each species and quality level. Anyone can go in and ask for there currant spec sheet. Just like firewood is sold by the cord, pulp is sold by the ton (or cord ), saw logs are sold by so much a thousand board feet. The mill I used last winter had on the spec sheet: Saw Logs - Spruce/Fir 10, 12, 14, 16 feet, 6" min top, $280 MBF. Which means they will pay $280 per thousand board feet of logs. The logs must be saw log quality, which will be described in the fine print, 6" or more in diameter at the small end, and cut at least 10'-4" ,12'-4", 14'-4", or 16'-4" long. To figure the board feet in a log there are several "log rules". We use the "International 1/4 Rule" here. It is a table that shows how many board feet of lumber can be cut out of a log given its diameter and length. For example a log that is 12' long and 12" in diameter at the small end should yield 70 board feet of cut lumber. So when you bring in a load they measure each log, find its board feet and add them all up. The load in the picture was 505 board feet which got me $141.40 (505/1000 X $280). I got paid once a week on fridays for all the logs I delivered that week.
That is basically how it works. Some mills welcome little guys like me and some only want to deal with big loads. Most yards take anyones wood, even one log, if it meets there spec. They often pay less because of the extra middle man involved. But if they are close you save on trucking.