Travelling to a no sales tax state for large purchases. Sales tax is roughly 10%, state with no sales tax is 150 miles away for me.

Doing the math, 300 miles round trip, 30 miles per gallon, $4/gallon for gas, if I'm buying something that costs more than $400 I get a free trip to other state.

Downside is that you're only breaking even for the time, but if you're making a $1,000+ purchase then it's definitely worth the time for me to make the trip.

Some states have that as a "you should/need to declare that as a use tax."

https://en.wikipedia.org/wiki/Use_tax

It's likely poorly enforced, but it's on the books and it's a complicated one to track. It was more of a concern when internet sales didn't collect state sales.

There's also Simplified Sellers Use Tax lawsuit that was recently in the courts.