Most states already use sales taxes for a significant portion of their income. Consumption taxes also have a long historic record of usage. I'm curious as you how/why you think people would conduct more goods/services "underground" than they already do to avoid paying state sales taxes.

This link (pdf) http://www.fairtax.org/PDF/Lamar_Smith_rebuttal_letter1-11-07.pdf talks about the enforceability questions you raise at the beginning.

And this one (pdf) http://www.fairtax.org/PDF/Excerpts_from_response_to_tax_panel-103006.pdf
has more information regarding both you inquiries which I think you will find informative.

The relevant portion from the second pdf in response your your first question:

Panel statement #7: The FairTax has never been enacted in the world and is risky and untried.

FairTax response: Consumption taxes are the oldest form of taxes on this planet; they require very little government overhead in comparison to direct/income/poll/head taxes. Phoenician traders paid a percent of each ship’s cargo for the privilege of using safe Mediterranean ports. The greatest expansion in the history of the British Empire followed the repeal of the detested, only-to-beat-Napoleon income tax with the subsequent use of consumption taxes. Florida and Texas, two of the largest economies in the world, rely primarily on sales taxes.

The FairTax is a pure consumption tax with a tax base virtually identical to value-added taxes used by 29 of the 30 OECD7 nations. Distinguished from these systems, however, the FairTax replaces all forms of income and payroll taxes, where these VATs are add-ons to existing income taxes. And the FairTax is a visible tax being separately stated on the retail receipt, whereas VATs are usually hidden in retail prices.

Based on 2005 VAT rates, the average rate is 20 percent with 14 countries having rates of 20 percent or more. Five countries have rates equal to or greater than 23 percent; again, these rates are in addition to their income/payroll tax systems (Belgium: 21 percent; Denmark: 25 percent; France: 19.6 percent; Ireland: 21 percent; Italy: 20 percent; Hungary: 25 percent; Austria: 20 percent; Poland: 22 percent; Portugal: 21 percent; Slovenia: 20 percent; Finland: 22 percent; Sweden: 25 percent; Iceland: 24.5 percent; Norway: 23 percent). The FairTax rate of about $0.23 out of every dollar spent is a revenue-neutral replacement for our income/payroll tax system