The final value fee for any ebay transaction is charged to your eBay invoice immediately at the moment that the item is Sold. Then if the buyer doesn't pay, there's a process for getting a credit for the fee.
When you are in a transaction with a member, you can use the Advanced search link for "Find cotnact information" to pull the member's info including their phone number. When ebay gets this request they automatically send dach meber's contact info to the other member.
It used to be easy to make arrangements using ebay messages, but lately ebay sellers have been getting warnings about arranging off-eBay sales. This doesn't make any sense when you are communicating through a listing that's already Sold!