As I am quite lazy (and getting lazier, it seems), I simply tick a box on eBay when I am listing a Glastonbury-related goodie that I don't need anymore and hey presto! It makes a donation to the charity of your choice from the sale. The options include the brilliant Marie Curie Cancer Care, and once someone pays for the item, eBay's partner MissionFish sorts it out for you.
My item is sold within about 5 minutes (which suggests I either listed it too cheaply or there is someone out there as excited about the world's great festival as I am!).