How will the SEO firm work on your site? Are you going to give
them access to your server, to where they're responsible for
uploading pages and making changes?

Or, will they have to go through a Webmaster, thereby slowing
down the process. Some SEO companies like to set up gateway pages on their own server and route traffic to your site. Other SEO
firms like to set up separate Web sites on their own server as a
means of building link popularity and routing traffic. If that's
the case, who will own those gateway pages or sites if you decide
not to continue using their services?

When the SEO firm owns the pages or sites, if you take your SEO
work elsewhere, that firm can simply re-route traffic to another
client's site. There's nothing illegal or shady about this-it's
just a fact of life. However, you can always choose to negotiate
ownership of a gateway page or site in the initial contract.