The buyer has 7 days from when they were notified that their item is at the collection point, to collect their item. After that they lose all eBay protection concerned with any refund claim.
If they don't collect in time, the store can deal with the item as they wish, send it back to you, give it to charity or even destroy it.
If you do receive the item back, personally I think as you have both the item then, and the money one has to go back to the buyer. Cancelling the order will refund everything the buyer has paid, and you can relist the item for sale.
Personal circumstance or mere forgetfulness, you may never know the reason for the item not being collected.
@baz8755