How to set up a trial-based account for users on an ubercart site?
Create 2 user roles: Trial Seller and Seller. 
Create a Rule. 


Event: "Drupal is initializing"
Condition: User has role -> "Trial seller"
Action: php that will hide (disable) the add to cart button. You have to use the hook_form_alter function (or similar) like described here: https://drupal.org/node/773312#comment-7425408



The second question is another seperate task.


Create a Rule:


Event: "Drupal is initializing" 
Conditions: User is of role "Trial Seller" AND not "Seller" AND php "(Current_date_time - Account_Created_Date) > 30*24*60*60" (to see if the time from the user account date until now is more than 30 days, time in seconds) AND he has paid
Action: Remove user role -> "Trial Seller"



There is also the module Role_Expire that may help you.