It's easy.

The C means cash, so people think-ah, cash when the item is delivered over to them as in handed over at pick up.

I know that is the wrong thing but I am sure a lot of people don't think it through, they just think-cash.

You're right though, it is best to take it off the options for payment.

I am not sure why ebay would even list it, there surely wouldn't be many sellers who would use it?