From the State of Illinois:
"In 1955, the General Assembly passed the Use Tax Act. Use Tax is a sales tax that you, as the purchaser, owe on items that you buy for use in Illinois. If the seller does not collect at least 6.25 percent sales tax, you must pay the difference to the Illinois Department of Revenue. The most common purchases on which the seller does not collect Illinois Use Tax are those made via the internet, from a mail order catalog, or made when traveling outside Illinois. You must keep your receipts when you make these types of purchases."
(see https://www2.illinois.gov/rev/Individuals/pages/Illinois-Use-Tax.aspx)
There are two important items here:
- Did the seller in Wisconsin collect sales tax?
- Did you enter on your Illinois return the amount for the Use Tax question in the Illinois state interview in TurboTax?
If the seller in Wisconsin charged Wisconsin sales tax AND the sales tax was greater or equal to the Illinois sales tax rate of 6.25%, then you don't need to do anything.
If you paid less than 6.25% sales tax in Wisconsin, then you owe Illinois Use Tax for the difference.
See
"Can I get credit for sales or use taxes paid to other states?
Yes,
we do allow credit for taxes properly due and paid to other states.
However, if you pay tax to another state at less than the Illinois Use
Tax rates, you must pay Illinois the difference." (see https://www2.illinois.gov/rev/Individuals/pages/FAQs-Use-Tax.aspx).
If you don't file an Illinois state return, but your hope-to-be other half does, then have your other half report whatever use tax you might owe on his/her state return.
