Bark beetles are probably the number one concern when discussing firewood and insects. If your trees are not harboring bark beetles, you can safely cut and sell them for firewood.
To see if there are bark beetles in your trees, peel back some bark and see if there are galleries etched in the underside of the bark. If there are no galleries or adult beetles or larvae, you are good to go. If you do find galleries or beetles present, you could still sell the wood, but you should cut and stack it, then completely enclose it in clear plastic for a season. This will "cook" any bark beetle larvae or adults that are still in the trees and stop them from spreading. If the trees have been dead for several years following a burn, you may find galleries etched into the underside of the bark, but the beetles are long gone. In this case, it is safe to sell those trees as firewood as is.
Other insects to be wary of are secondary borers, or insects that eat dead wood. Firewood stacked near or next to wooden buildings could be attacked by these insects. To prevent this from happening, stack firewood away from your home and outbuildings in an open-air shed.
