Hi Nathan, in answer to your questions:
1. To clarify, AppThemes licensing allows for developers to develop websites for clients using our themes, however purchasing a website with the theme installed does not automatically make you an AppThemes customer. This means you will not have access to theme updates or access to our support forums as our customers do, so if you experience issues with your theme, or are in need of an upgrade you would need to approach the developer for further help. You also cannot always be sure that the developer you are dealing with has legitimately purchased their theme in the first place.
2. As mentioned above, if you're not buying the theme yourself from AppThemes, you are essentially left to rely on the developer for future support relating to the theme.
3. As also mentioned above, buying a website with the theme installed will not automatically grant you the same access than an AppThemes customer has, so no, you would not be able to access updates from AppThemes unless you actually purchase the theme from us.
4. You wouldn't share your AppThemes login credentials, but yes you would likely need to grant them access to your website in order to do the work they need to do. You can of course set up multiple accounts in WordPress for this so they have their own credentials. If you are working with one of the
AppThemes certified partners for your development work, then you know they'll be a reliable source. We can't really speak for other developers in terms of their trustworthiness, but we would recommend you do your homework before making a selection. Thanks.