You can't exchange contact details with the seller until you've placed your order.
However, you can ask the seller questions and negotiate via the website. To do that, follow these steps:
1- Sign in to your account
2- Go to the listing of the item
3- Go to the "Comment on this item" section at the bottom of the page
4- Type your message and send it
All comments are moderated before they appear on the website. You will receive an email notification when the seller replies to you.