Earlier quoted context omitted.
Can you please go with me to the Seattle or Bellevue Apple Store and help me make this transaction? I went to both over the weekend and things did not go well. They didn't seem to understand the basic process you outlined above.
All phones sold at Apple stores are unlocked, but they come with an unactivated carrier SIM. I purchased an iPhone 7 (t-mobile), then popped out the SIM and put in my old AT&T sim. No problem. I suggest just buying the t-mobile device and doing the same, if you're looking for a GSM device.
Seriously though I showed up at Apple with a credit card and a SIM card 48 hours ago and they were not able to sell me a phone. To make it even weirder, I had the (unopened) phone I wanted in a bag, but it is a "locked" one from TMobile. I thought that was ancient history.
One day Apple will want my money!