This is interesting... I don't know the particulars about this, but...
It would seem to me if the buyer pays, it would be a sales tax, if the seller pays, it would be an income tax. Two different rates. I don't know of any state that has a slaes tax higher than it's income tax (if it has both taxes).
Depending on who is doing the taxing, an income tax could be levied by both the state (the seller lives in) and the Feds.