J1m,
A face cord is or should be a standard for everyone.
4' x 8' x 1'
Some people have 16" but one standard face measure is 12"
If you have 16" wood then that face cord is really 1.33 face cord or 1/3 of a full cord.
We really need some set measure since some people burn 12" wood and others up to 24" wood.
Tough to measure apples to apples that way.
20 pieces of wood to person A can be quite different than 20 pieces of wood to person B.
Both are fine to sell here since it's 1/4 or 1/3 full cord and a customer would be told he or she is buying not a face cord but 1/3 or 1/4 cord.
With those numbers a face cord now really is meaningless but more of a measure that has become a standard broken into a more reliable total.
In Canada your only legally allowed to sell some % of a cord.
Not a bad practice either since seller and buyer agree on an amount of money for an amount of wood with no wiggle room for either.
Selling something else here will probably get you a warning, continuing to sell it will get a fine.
Even for me a face cord is more of a habit than a meaningful measure, when a customer asks for one I catch myself and ask if they would like 1/4 cord or 1/3 at $ or $.
Now with a useful measure we can say I burn 1/4 or 1/3 cord in X time and it will be a useful total for everyone to match against.
Then burning 12" 16" or 24" will just be a total of what percent of a cord you burn in X time easily matched against person A or B.