Thanks Chris!
It would of course also be a great way of getting rid of SIM's that are going to expire anyhow. Who knows how many Turkish, Indian and other SIM's I have had to let lapse in this way.... all of which could have been some good to somebody. I don't know how we would figure out partially used credit?
Also some of us who travel a lot have established easy places to buy a SIM card. These experiences may be difficult first time round but once you have done it once, you know the ropes and it is easy picking up another SIM in that shop around the corner from your regular layover hotel, local supermarket etc. Perhaps next time you are travelling you will have a few minutes to kill and remember that request from a ppgsm buddy.
As it stands at the moment, I may not actually have that many SIM's to hand, but I know exactly how to cheaply and easily lay my hands on many like these the next time I travel.