Marketing is about communicating with larger groups of people - first by getting the message to a large crowd and then working to separate it into smaller subgroups and provide more targeted messaging. Sales go on a deeper one-to-one level to complete the transaction.
Marketing can take up a lot of forms that include writing content and optimising SEO, through analyzing AdWords campaigns or working on more creative projects.
Marketing skills are useful for your general business understanding, but will they will be highly applicable for an outside sales career? To be honest, I think they will be more useful for an inside sales rep, but it really depends on the job and on the company.