In Illinois they charge you what you paid or market value. Unlike the FFA who doesn't care, if you report 1+OVC they'll happily value your plane, sight unseen, and send you a bill. I was happy to tell them what I actually paid, since I think I got a deal.
The tax itself was actually a really big consideration. I calculated the max I could pay including tax. When I called to look at an airplane over my max (only when within 2k) I was honest with the owner. If he wouldn't consider my max bid, I wouldn't waste his time at the airport.
I'd also be wary of not paying the tax for insurance reasons. Many policies don't cover if you are flying in violation of any state or federal law. If a state requires registration and you purposely under report the value of your plane, one could say that your registration was fraudulent, you do not have a valid state registration, and that you are flying in violation of a state law.
Just because it doesn't usually happen, doesn't mean it can't. Especially if you've got a jerk adjuster.
I'm not saying 1+OVC isn't a valid entry on a bill of sale, just that I would honestly report the sale price to the state for tax purposes.
Now I shall await a tirade of criticism.