Generally, online shoppers know that they are not talking to professional customer service employees but to members of an expert community.
If you should feel like the customer you are talking to may not be aware of this fact, you are encouraged to friendly remind him of your position.
Please be aware of the fact, that the customers have the possibility to receive the course of conversation per e-mail if they wish to. Naturally, that allows them to re-read your choice of words and attitude.
It can happen that a customer leaves the website before your conversation is over. Don't take it personally. The messages you will send afterwards may not be read immediately but will be delivered as soon as they come back to the website.