Getting a second hand phone that’s carrier locked doesn’t make sense to me. The original owner didn’t unlock it? Usually phones are carrier locked because you’re financing the phone through the carrier, but that wouldn’t apply to a used phone.
Sounds like you encountered an anti theft process. What would keep people from just taking a stolen phone to t-mo to have them unlock it without this? Sure you could give them a receipt or print out the eBay listing etc. but those are easily faked. Nothing seems out of the ordinary with regards to what T-mobile did to me. You need to make sure the phone is unlocked before you buy it. It’s been this way since I was selling phones at RadioShack in 2006.