I am trying to find out when a county can sell property under foreclosure for unpaid property tax (on land and house)? It is Mercer County in West Virginia specifically.

Hello curious:

IF the county foreclosed (as in past tenths), they can hold a sale anytime. If the BANK foreclosed, then the county can't sell it at all.

excon

If the property was foreclosed, then it is owned by the bank who forclosed.
If the owner still has it, and the county is going to do a tax sale that is different. On a tax sale each state has its rules, normally it is two years back taxes owed, some has 3 years, it would or could be legal to sell it at one year