Does it is legal to buy iphone feom customs

What customs?

It's legal, but it may not be a good idea. If you are thinking of doing this you need to check that the phone is not 'Activation Locked' - this won't show up until you reset or upgrade the phone if it's enabled, and at that point it will render the phone absolutely useless. You can check this by going to https://www.icloud.com/activationlock/ and entering the phone's serial or IMEI number.

However if it's been impounded by Customs and is still in the original packaging and not unwrapped it probably is OK - it's when there has been a previous owner using it that the problem arises. If it looks as if there was a previous owner and you cannot confirm that a phone is not Activation Locked you should not buy it.

I don't know whether you will get any sort of warranty from Apple even if the phone is unused.