As an SEO specialist we need access of the following:
1- CMS Login details (If the seller is offering on page SEO services and let's say you have a WordPress/shopify website, You need to provide him login access details for your WordPress/shopify website, as the job can only be done if the seller has the login details).
2- Access to Google Search Console
3- Hosting (If you need website speed optimization, seller might ask you for hosting details as well.
Please feel free to ask, if you have more queries:)
Thanks