Rather than operate on the usual ‘first come first served’ basis of retail, Banksy decided to open a kind of lottery cum merit-based system for collectors wanting to buy his work. In order to get your hands on one of his pieces, you had to go to the GDP website and ‘apply’ for a product/artwork.
An attempt to fulfil the trademark requirement to sell clothing with the artists name written on – these tagged tees are made out of charity shop shirts vandalised with stainer ink, spraypaint or oil stick. Every one is different, will last a while but might stain other stuff if you wash it at the same time.
More information here