You are no longer allowed to accept cash on collection and should not be able to exchange contact details until the buyer has bought the item and paid through Ebay checkout.
It doesn't sound like a scam - just a buyer who doesn't know how Ebay works.
"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)
