Ask news.yc: Taking payments for third parties?

Hi all! I'm considering an idea that involves taking credit card payments from consumers on behalf of my customers. I'd need to collect the payments then, after the service had been supplied, send it on to the seller (by cheque or bank transfer probably). I'd be basically acting as an online storefront. Does anyone have an experience of the problems I'm going to encounter here? I imagine there could be problems with liability when holding other peoples money or issues when refunds need to be made. I'm based in the UK so I'm mainly interested in how it works with the laws here but I'd presumably want to expand out at some point so any advice about elsewhere would be useful too. All advice will be much appreciated, thanks!