If you cannot collect within the timeframe that the seller has specified ask somebody else to collect the item on your behalf. In order to do this look for the e-mail that eBay sent you with the QR code, copy and paste it onto a sheet of A4 paper and print it out. Give it to your helper to pass on to the buyer when he or she has collected the item on your behalf, and that way the seller can scan the code and receive the payment for the item. Once you return your helper can then pass the item over to you, in which case the fact that you won't be around on the date that the seller specified shouldn't be a problem.