Slight design problem with Roles, Rules and Profile2 on my site
Here's a fairly straightforward way to do it.


Create a field (boolean) on the user account manage fields page.  Assign it useful label such as "I want to run a store" etc.
Create a rule to add the role as follows:


Event After saving user account AND After updating an existing user account

Conditions Data comparison (user account field value) == yes AND User has role (store) NOT

Actions
Add user role (store)

And create another rule to remove the role:

Event After updating an existing user account

Conditions Data comparison (user account field value) account-unchanged == yes AND data comparison (user account field value) account == yes

Actions Remove user role (store)

This should let the users check the box and then the role will automatically be toggled on or off.