I have an idea for a web site but it involves allowing people to use my site to purchase goods. My payment details (transparent and securely) are submitted to the site which they wish to purchase from, while their personal payment details are submitted to my account, equaling the cost of the purchase. I need to find out how posible and secure this is. Scenturian...
Your site just needs to take the same precautions that any other conscientious secure site takes (think your bank, Paypal, etc.). You think of yourself as a middle man, but you're just the recipient for the one transaction and the originator for the other.
We already have Google Checkout and paypal. Is it you are making such thing for learning or for doing some real business?