As someone who did this trial day, I can say that they seemed like nice people. This proves nothing in this day and age, unfortunately devoid of trust and good spirit. They did indeed take my ID (driving licence) and i got it back at the end of the day without hitch.
As for people claiming that this is commission based, I can confirm that this is indeed false. I don't know about other charities, but shanti international pay 6.19 £ hour-1. The condition that you pass the trial day is that you make more than £6.19 every hour, armed with nothing more than a cheap shanti international ID card, a large purple bucket (sealed, of course) and your own wit and guile.
Of the 6 of us that started the trial, I believe only three made it through the whole day. It was hard, cold work, and you have to put up with a lot of people ignoring you. Some people just aren't cut out for it. You will need a thick enough skin to affect a personable demeanor even though very few people give a damn about you, and not to care too much about the cold. They haven't contacted me back yet, although they did make a point of taking my number before I left, as well as the days I could work (this was on Saturday).
All in all, I see no reason that you shouldn't go for it, if you think you have the balls to do the job, and do it well enough for you to be worth their employership (is that even a word? a question for another time.). Having met them, they do not seem like the shady sort, but stay on your guard nonetheless. Basically, the work's hard, the pay is low, but it's as fun as you make it and you get to meet a lot of people, both fellow trialists and members of the public.
TL/DR:
Successful applicants are paid 6.19 £hour-1
The ID-taking is to prevent you stealing their buckets
It's really hard
It's really cold
It's as fun and as rewarding as you make it.
Well, that's just my opinion and experience on and of the matter. I hope you find this useful.