How can I switch career from sales to Finance?

I am only giving you my educated opinion from what I know, however I am not familiar with your region; so this may or not be applicable.

Since you already have an MBA in finance / marketing you need to gain some experience in the finance industry. You may be able to get started in an accounting position. It is a tough market right now from everything I have read so getting your foot in the door even if it's an entry level position can't hurt. Once you prove yourself you should have no trouble moving up in the organization or even seeking employment in a similar role elsewhere.

You can go after the CFA/CMA/CPA but at some point you are still going to need the experience.

I completely agree with Christie. Any potential employer will want to see real life experience. There is a very big difference between finance in theory which academics provides vs. finance in practice which work experience provides. A little bit counter intuitive, but additional finance education for you will have diminishing value.