Buying used iPhone 5s for use abroad
If you buy a locked phone, then it will be locked to the carrier it was originally bought for. But I guess you know that already.

I am assuming you are in the US, since you mentioned Sprint.

There are two things you can do:


Buy an iPhone that was already unlocked.
Buy a locked iPhone and use it as a phone in the US, but buy a second phone for use abroad. This can be bought abroad or in the US, but make sure it is not carrier locked. When travelling abroad, you could use the iPhone wherever you have access to WiFi - and you could use the other phone for making phone calls. If this phone is also a cheap smartphone, you could even use it to tether the iPhone to it for WiFi access on the move.


Buying an already-unlocked iPhone is not the cheap option, and I assume this is an issue since you were looking on eBay for a phone. But you could get a low-spec Android phone for very little money if it's a few years old. The key thing is that it would have to be unlocked since you would be using it for phone calls abroad, and possibly data access if you use it as a hotspot.