I’m about to sell my X to a coworker, does it matter whether I remove the sim card or leave it in when doing the factory reset?
You must disable "Find my iPhone" feature before you do the reset. SIM doesn't matter. A good reference for "clean" reset for the purposes of reselling/trading phones is here:
https://glyde.com/glydecast/how-to/wipe-iphone/
It’s habit of mine always log out of my iCloud account before factory resetting. By logging out of iCloud that disables find my iPhone correct?
How much did you sell the X for? And size ?
Sold for $700 to my coworker. Mine is 256 GB