I think receiving money by the sale of an ethically tainted property constitutes complicity.

For example the seller in a slave trade is complicit of the slavery that is going to continue of being imposed on the victims by the buyer.

The ethical attitude is to liberate the slaves by manumission.

If we accept the premise that any coal mining is unethical, then the owner of the land should "liberate the land" from the coal miners. That means he must keep the mining rights, and refrain from selling those rights to coal mining companies.