You don't want to be an MSP unless you're doing enough aggregate transaction volume to manage the risk adequately. You can become a reseller for authorize.net

You don't want to be an MSP unless you're doing enough aggregate transaction volume to manage the risk adequately. You can become a reseller for authorize.net or join any of a number of affiliate programs for payment gateways but you should plan on having to walk your customers through the approval process themselves. If your customers aren't large enough ( > $2000 USD/month txvolume ) to get their own merchant accounts you are stuck with one of amazon, googlecheckout, paypal or possibly getting your own merchant account. Be sure you understand the risk you are taking when you start handling other peoples money, you will be liable for non-delivery and fraud from your customers and your customers customers.