An SEO Reseller oncharges the SEO work of other companies to their clients.  Often this is "White Labelled" and the client doesn't know that the company they are paying to do their SEO, is not actually doing it themselves.  

Digital marketing and advertising companies or agencies typically use SEO resellers, who do SEO work for your business, which is then passed off as your agencies' work to your clients.
 
They are a great option if you own a marketing company and need to expand your line of offerings to offer SEO services to your clients, without hiring full-timers or freelancers, which can be costly or hard to manage. You can put in a free quote here to find SEO resellers in Australia