You have asked for cash on collection.
This is no longer allowed on Ebay - if you accept cash, there is no way to pay the new Buyer Protection Fee to Ebay, and the buyer has no right to return a faulty item.
Once the buyer has paid through Ebay checkout, you will be able to send your details.
To be honest, Ebay is no longer the site for selling if you want 'cash on collection'.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)