I am not sure how does it help the seller not to pay taxes if you live in a country where you don't have to pay taxes? It's all about the seller and his personal tax situation. If he makes a profit on his bitcoin transation he will have to pay taxes - sure he can try to hide it and avoid paying taxes but that would be illegal.

Or does it mean you would pay the taxes for the seller with a higher price? But then you could just buy on an exchange outright. Maybe I am missing something here but everyone needs to check for himself and his tax laws.