SEO resellers are companies that offer SEO services to clients through outsourcing. They partner with established SEO agencies and provide SEO services to their clients, while another agency handles the actual work. This is called White Label SEO Services An SEO agency in Canada named as SEO1st also provides white-label SEO services in Canada. If someone is from Canada they can contact SEO1st.
An SEO reseller is a business or specialist who helps other marketing agencies with SEO-related tasks so that the agency can offer their clients a package which includes search engine optimization even if they don’t have their own in-house SEO team.
SEO is a broad field of study so the exact nature of the tasks they carry out will differ from company to company, but it always involves helping to optimize a webpage for search engines so that it can rank higher in the SERPs for the chosen keyword/keywords.
This may involve building backlinks, creating content, finding keywords, optimizing site structure, creating appropriate image tags, amending existing content, or lots of other possible strategies.
A practical example of an SEO reseller may help you to understand what they do. Whitelabelers Ltd is a SEO reselling website, so you can get a better picture of exactly what they do if you browse their website.
I hope this helps!