Explanation
Sales users are Marketing Cloud Account Engagement users who have access to the prospect database and can send one-to-one emails to prospects, but not list emails. Sales users can also view and edit prospect records, create tasks and activities, and sync prospects with Salesforce. You can set up sales users in Marketing Cloud Account Engagement by assigning them the Sales role and enabling the one-to-one email option in their user settings. References: [User Roles], [Create and Edit Users]

A completion action is a type of action that allows marketers to define what happens after a prospect completes a specific activity, such as submitting a form, clicking a link, or opening an email. One of the capabilities of a completion action is to remove a prospect from a list. This can be useful for managing list membership, suppressing prospects from future sends, or moving prospects to different lists based on their behavior. A completion action cannot send a list email to a prospect, increase a prospect's grade, or adjust a prospect's account field2. Reference: 2: Completion Actions

Form handlers give you total control over your form's appearance, since you can use your own HTML and CSS to design and style your forms1. Form handlers also allow you to use any custom or advanced features that Marketing Cloud Account Engagement forms may not support, such as file uploads, conditional logic, or complex validation3. Form handlers are ideal for web developers who want to have full flexibility and customization over their forms

According to the Salesforce documentation, the marketer can accomplish the goal of creating different Marketing Cloud Account Engagement lists to correspond with the different stages of the buying cycle and automatically updating the prospect list membership based on the opportunity stage change in Salesforce by using a dynamic list. A dynamic list is a list of prospects that is updated automatically based on certain criteria, such as prospect field values, activities, or scores. A dynamic list can be used to segment prospects based on their opportunity stage in Salesforce, and to add or remove prospects from the list when the opportunity stage changes. For example, a dynamic list can be created for prospects whose opportunity stage is Negotiations, and another dynamic list can be created for prospects whose opportunity stage is Closed Won. When an opportunity moves from Negotiations to Closed Won in Salesforce, the prospect associated with that opportunity will be removed from the Negotiations list and added to the Closed Won list in Marketing Cloud Account Engagement. A completion action, a page action, or an automation rule are not the best tools to accomplish the goal of creating different Marketing Cloud Account Engagement lists to correspond with the different stages of the buying cycle and automatically updating the prospect list membership based on the opportunity stage change in Salesforce, as they are related to other aspects of automation, such as performing actions after a prospect completes a marketing element, visits a web page, or meets certain criteria, but not segmenting prospects based on their opportunity stage in Salesforce. Reference: Salesforce documentation