I have an iPhone, but no carrier. Is it unlocked?

I've had this iphone 4 for a while. I cancelled my service a few months ago and kept the phone, now I want to sell it. If I sell it to someone, I assume they can use it with whatever carrier they want, so does that mean its unlocked?

You have to make sure that your carrier when you cancelled your service with them had unlocked the phone for you.

Its still locked to your carrier, and might be blacklisted if you didn't pay to get it cancelled.

Call your carrier to have it unlocked, if you finished your contract then they will unlock it.