The negatives are supplying things like cheap paper to the post office for marketing departments to across to spam people using US Postal Mail. _Thats_ what needs to be stopped, as it's just a waste.
Absolutely not. The amount of carbon sequestered is inconsequential relative to the habitat destruction, habitat fragmentation, species going extinct, etc.
Each pound of lumber represents 1.8 pounds of carbon dioxide our of the air. It looks like both logging trucks and flatbed timber trucks can carry around 24 tons of lumber, which would be 43 tones of carbon dioxide.
Sawmills are efficient, and would make a negligible contribution to total carbon. Transportation would be the biggest carbon cost. At least on the East Cost US there's a lot of timber growing so distances aren't too far. Let's say 200 miles total from logging to sawmill to use. 400 miles round trip. Using average US freight truck emissions that's about 0.07 tons of carbon dioxide released.
So let's round that up a bit and say 43 tons sequestered for for 0.1 tons expended. Seems like a win.
Unless I'm really off on my math.