Serial entrepreneur (3 exits), start-up investor and digital marketer to the affluent
I've sold my digital agency before and I happen to know a few things about this.
First, you'll need to know that there are three main types of buyers for your SEO company.
1) A bigger agency
Some agencies (eg. content marketing agencies, creative agencies, social media, or even lead gen agencies) say that they can offer SEO services, but most of them don't really have any in-house expertise. If your agency is already profit-making, then it makes a lot of sense to sell to a bigger agency.
But you'd want to be careful of how the deal is done. Most agencies acquire smaller ones for a P/E multiple of 3x to 5x with lock-in and earn-out terms. That's as good as taking a loan from the bank and paying it with future profits from your agency, so you'd want to avoid such deals.
2) Selling to your clients/brands
Some brands/existing clients may need your team's services as a long-term plan. It's easy for them to hire an SEO agency (which will be a cost in their P&L), but if they'd acquire yours, they would be able to break even on their investments. In my case, this was my deal when I exited my agency.
3) An incoming player
There will also be senior marketing professionals who'd want to get out of their corporate lives and run their own company. Given that marketing is the only thing they'd know, some of them might be interested in buying over an agency with existing clients and operational processes, so it'd be easy for them to continue/scale up the business from there.
I recently just wrote something about this here -- https://www.searchenginejournal.com/exiting-your-digital-agency/264551/
Hopefully it helps! Let me know if you'd want further advice. This was a path that I've gone down before and I'm happy to share anything that could help you make a wiser decision.