Cash is not allowed for collection items. If you look at listings for collection there is no option for cash as a payment.
Not sure how your buyer managed to get your contact details, as this would usually only be available after a buyer paid at eBay checkout.
You will not be able to mark this as collected, the worse that can happen is the buyer, if not honest, can open a case for item not received, and you'd have no proof it was collected.
@demondriverdave