I did just this as I am traveling to Asia in a few days.
I opt'd to sign up for the month to month Simple Choice plan from T-Mo since it provides free txting and unlimited data (2g) over seas.
I used an online unlocking service to buy an unlock code last month. You provide the IMEI, and payment, they provide a lock code.
Once you have the code, purchase your new nano sim card (straight talk, t-mobile, out of country provider) Turn off your phone, swap sims, and turn on your phone.
Upon booted up and back to the normal lock screen, it will ask you to input the code, at that time you put in the code, phone should unlock no issues.
To my knowledge this is where its dicey for 2nd hand phones on craigslist. I believe there is a set limit to how many times you can wrongly input an unlock code. From what I read its five to ten tries; seems no one can confirm. But inputting the wrong code too many times will lock itself up; rendering it useless; and I do not believe it will revert back to give you anymore tries.. So if you guys a phone on craigslist, the original owner tried to unlock with bad codes, it will count towards the miss tries. So you don't have a clue how many times you got.
But to go back, T-Mo works well for me in Scottsdale, AZ. call quality was fine, i was surfing the web without an issue on an AT&T Moto X.