Long time lurker here, came across this post and decided to make an account as I can probably answer a few questions.
I used to sell the big issue around 4/5 years ago when I was homeless.
I had to pay a deposit for the red vest that you are required to wear, it cost around £20.
You need to have the big issue photo ID, if you don't have it you will be I trouble.
you buy the magazine for half the price its sold at, so i paid £1.25 per copy and sold them for £2.50.
The magazine changes every Monday to a new issue, if you are caught selling last week's issue you will be in trouble, if you haven't sold the copies you nought by the next Monday then you need to bin them.
There is a map of pitches, you get given a pitch to stand at and you must stay there, you will get in trouble if you go to someone else's pitch to try and sell.
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There is a rule that you don't take any tips or keep any of the money offered apart from the £2.50, but the people who ran the big issue in my city never gave people trouble for it.
And lastly I always gave change straight away, I never assumed I could keep it no matter how big or small, but yes if someone told me to keep the change then I thanked them and kept it.
Oh and I also always gave the magazine but it did pain me when people took the magazine and then instantly binned it.