Dried up wooden building material is pretty stable and a fairly common (even commercialized) suggestion for a carbon sink. If you let it rot/burn, then no, it is not a carbon sink.
A 25 year old maple will sequester 400 pounds of co2 in it's lifetime. An average commuter car will emit that in 2 weeks. Hauling all that carbon out of the forest will likely emit far more than that.
If we're building the buildings anyway, we have to compare with alternative building materials. That is, how much more would transporting that lumber emit, compared to transporting for instance brick?