Typically, a lumber company will want to purchase rights over an extended period, so they can harvest prime lumber and give some other woods time to mature, so you wouldn't get an ideal price if you require an immediate clearing.
Also, if you were in a situation where you may be able to make money off of timber rights (many hardwoods that are usable), I'd suggest that the owner of the property may have already availed himself of that revenue stream.
As Tom mentioned in the link, any lumber revenue would merely be an offset to the actual clearing costs for the items that you can't sell (roots, non-hardwoods, less than mature trees).