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.
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.
I have in fact brought a rolled up full size mattress home in the back seat of a Fusion Hybrid (it fit! with room for other things!) and it was a great cost savings. As a bonus at the time there was an additional sale in IKEA CA on the mattress that US didn't have, so I saved even more.