Do I incur a sin by purchasing an electronic shisha for a friend, and what should I do with the accessories I bought but haven't sent yet?

A fatwa was previously issued prohibiting e-cigarettes containing nicotine and harmful substances, and the claim that they help quit smoking is a lie. You bear no sin for purchasing them while ignorant of their ruling, as ignorance is one of the impediments to accountability. As for the accessories you bought and have not yet delivered to the buyer, if it is possible to cancel the sale, then do so. If not, there is no blame upon you for delivering them to him, while advising him to abandon this evil and clarifying its prohibition.