As a private seller you don't have to accept cancellations or change of mind returns.
However, the buyer can open a money back guarantee case for a full refund if the item is even one day overdue. I'm afraid you are responsible for refunding for undelivered parcels, and even for items that arrive late, if that's what the buyer wants - and you would have to pay for the return.
Plus of course although you don't have to accept change of mind returns, buyers can easly claim that an item doesn't match the description in some way. eBay's rules are so weighted against the seller that the buyer is almost sure to win.
These are the realities of selling on eBay. Now that eBay charges buyers for buyer protection they will feel even more entitled to use it.