Until the sale is completed, you can only communicate with the potential buyer by using the comments that are shown on the product page. You can also find these questions/answers in your shop under "Messages". You cannot use these comments to pass on your contact details (phone no, address, etc.).
If you have confidential information to pass on to the customer, please do it via email@example.com.
Once the sale is confirmed, you will find the buyer's contact details in your shop under "Sales" and then "All my sales".