It's just a question of asking nicely. No reasonable person would refuse a polite request to use a tool if you stay within their shop and especially if you buy something from them first. Everything in life is negotiable and every man has his price.
Edit: come to think of it, we did exactly this on C2C last weekend when my son's cassette was loose. I asked the bloke who was escorting another group we kept meeting and he was quite happy to bring out his toolkit for me to use.