| Having purchased many a car in the great state of IL, I can say with confidence that the sales tax that you must pay the state is calculated based upon the prevailing rate at the location of registration. If you buy a car from an Illinois-based dealer, they will calculate the appropriate tax based upon the address to be put on the Title paperwork, which is where your car will be registered. If you purchase a car from an out of state dealer (i.e. in my case, Missouri) the dealer will typically not collect the tax, but when you go to get the license plates from the Secretary of State's office, you'll have to fill out a application for title and registration, and write them a big check for Sales Tax. They will base the tax rate calculation on where you live.
Believe me, I've done it several times and it never gets any better.
BigBrownDog
Collinsville, IL (St. Louis, MO Area)
MCS Chili Red/White | 
