Well, the FairTax argument goes something like this: evading income taxes only needs one actor (the income earner). I’m aware, by the way, that the employer can conspire with the employee as well. On the other hand, FairTax evasion will always require two actors: the seller and the buyer.
Before enacting the Fair Tax, I think we would need to amend the constitution to forbid income taxation at any and all levels of government, from the town to the UN.