You have just answered your own question.
You can not exchange contact details before the item has been bought as it is against ebay policy
Ebay do not allowe it
If it is a collection item then you buy it and then exchange contact details.
If it is not as you want then you and the seller should agree to cancel but they dont have too
If you do exchange then ebay may think you are doing an off ebay sale and that is suspendable or you get restricted
