As advised, sellers are not allowed to take cash on collection.
Repeat your conditions to the buyer, stressing that they will be responsible for collecting the item and ask them if they would like to comply with your conditions or cancel the sale.
"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)