Looking at your listings, CASH ON COLLECTION is not an acceptable means of payment. That is why payment should be paid before collection, as buyer gets a code to give you which proves delivery, and you cannot exchange addresses until paid.
If paid in cash there is no way to mark as paid, and you run the risk of problem of buyer claiming not received. As payment would not be shown as paid there would be no risk of having to repay, but buyer could prove to be a nuisance as far as feedback goes.
It will go a lot smoother if you only accept approved methods of payment, and follow the system.
