Buying an iPhone secondhand/Confused about carriers?

I'd like to buy an iPhone 4s/5c secondhand on Ebay or Kijiji or something but I'm really confused about the "carrier" thing. Does that mean that if the carrier is Rogers I can only use it with Rogers? I'd want to use it with Bell cards or Fido cards or something. Someone please explain in detail!

Is better to buy phones that are unlocked, meaning you can use any network's sim card on it. Unless you want to use the only network the phone is locked to.
Or you can buy phone locked to any network but you to unlock it for a small fee for it to accept any network.

Simpy look for the word "outright" (or unlocked), which means the phone is not locked to any carrier and can be used with any carrier. You still may want to check what carriers the phone is compatible with, as SOME phones may not work with certain carriers (do a quick Google search and all outright/unlocked iPhones should work with any carrier just fine). Also, you can always get the phone unlocked at a mobile repair shop if you still decided to buy that locked phone. This can cost around AU$50.

Yes, carrier iphone means you can use it with only one carrier, like if you buy at&t iphone then you can use it with only at&t, if you want to use with other carriers you have to unlock it. So either buy a unlocked iphone or you can unlock it after buying.