You can sell SEO services to local businesses by improving their presence in local search results. It would include an optimization process of the business website. You have to work on its reputation and optimize your Google profile. You can provide the following services to local businesses that will help them rank on Google:  

• Website localization 
• Local content creation
• Local link building
• Google Business Profile setup and optimization
• Google Business Profile Posts management
• Google Business spam fighting
• Building local citations
• Managing local citations
• Reputation management 
• Monitoring local search rankings
• Local search audit
• Keyword research

Nonetheless, this could be a long process, but before that, you have to prove yourself as the best marketer by providing facts and figures to your claims. Don’t make fake promises, and do your job by managing expectations.