Would I need a license to sell banners at a road race?

I was at a marathon today and I saw a few people who had made banners for their friends that were running it and I was thinking that it would be quite a good idea to take a load of blank banners and marker pens and sell them to people who hadn't brought their own. I'm 14 by the way. Would I need a license from the council, would I need to get authorisation from whoever was organising the race or could I just go and sell them. Thank you very much.