I know why this is happening:
There are lots of dodgy people on eBay setting up multiple accounts in false names with the intention on listing loads of expensive items for sale that they don't actually have, they sell the items, take the money and then forget about that account and move on to the next one.
To do this successfully they most first build up a good feedback score usually at least 50 positive feedbacks, and one of the easiest ways to build up fake feedbacks is by buying up loads of giffgaff SIM cards for 1p each. Because the giffgaff sellers have the address checked when they complete the orders, the dodgy eBay buyers have to use genuine addresses that check out ok.
Hence why some folk are getting giffgaff SIM cards through there letterboxes in the names of complete strangers, it's bad for the folk getting the junk mail, bad for folk trying to get plenty of activations by selling SIM cards on ebay and bad for giffgaff.