Haha, I looked into doing this for an avionics upgrade for a light aircraft I used to have a share in. Apparently the taxman actually has time to chase down people who fly their aircraft around for tax reasons.

To be clear, we wound up doing the upgrade in California where the aircraft was based.

I think state governments go after large purchases, like (pricy) wedding rings, aircraft, cars, or anything like that simply because that's where they'll get the biggest return.

It might be an urban legend now, but I've heard of a soon-to-be-married couple purchasing a wedding ring in Portland, OR, and making several trips from SF to Portland. They took photos of the ring at typical Portland attractions (Mt. Hood, Multnomah Falls, food carts), and did that for several months. Doing that, after adding up all the flight/hotel tickets, was still cheaper than buying the ring straight up in California.

At least a few years ago there was some bogus California tax thing where they added some money your tax bill assuming you were buying things online and other places without paying sales tax.

That's just a convenient, estimated option for if you choose not to keep track of your untaxed purchases yourself.