Yep, you can definitely cancel your buyer is being a tad / totally unreasonable, and should have asked before buying if this long delay in collecting was acceptable.
Send the buyer a polite message that 7 days is the usual length of time a seller is expected to hold a collection item. Now that 2 weeks has passed an no exact date has been given you need this item collected in ** days . If this is not possible you have to cancel the order which will fully refund them.
If you need to then cancel, click on the item in Orders click top left click on view order details . On the right from More Actions select Cancel this Orde, selecting the reason... Problem with Buyer's Account or Buyer Requests to Cancel. This will refund your buyer and your seller performance is not affected.
I would also then block your buyer from returning and messing you about like this again.
https://www.ebay.co.uk/bmgt/BuyerBlock?
@maju4661