There's an error in the T&C's for the free £10 goodybag with the purchase of a preowned phone offer, term 5 states:
"5. The 30-day return period is only applicable to Qualifying Purchases made from eBay.co.uk."
as qualifying purchases can only be made through the Marketplace this term is confusing and redundant, and should be deleted.
From here https://giffgaff.com/terms/promotions#free_10pound_goodybag_july2018