Depends what you mean by "legal". They're probably not going to clap you in prison for buying or selling a jailbroken telephone ... unless the jailbreaking was done in order to make a stolen phone usable by someone other than its rightful owner, which would be a separate offense.

It's always a judgment call. Years and years ago I opened my first computer to install more RAM, fully knowing that this would void the warranty. (I did say it was years ago.) I reasoned that by that point the computer was more likely to suffer due to insufficient RAM than to have an eleventh-hour warranty-related malfunction.

Someone is selling a jailbroken iphone in lower cost than original price. It seems to me risky by its name(Jailbreaking), so i was confused about it.

Hi uditsh, welcome to webmasterworld!

If you search 'Jailbreaking and Unlocking' you'll find several explanations.......