A really simple PayPal plugin.
Firstly, please check your SPAM folder. It's pretty common for WordPress generated email to get sent to SPAM, or even rejected by the recipient mail server entirely. A good solution is to configure WordPress to send systems generated emails over SMTP instead of using PHP mail(). I recommend Callum Macdonald's plugin WP Mail SMTP
You can sell whatever you want. There aren't any fancy features in this plugin - like it doesn't have delivery options, order management etc so it's really designed for just a single simple product. For example I use it to sell access to a private site - after successful checkout user is sent the password to log in. You could use it for other similar things like to sell a PDF or eBook, or you might use it for selling a virtual product or service like say consultation services for example.
Yes the plugin can have any number of products. In this free version you can only set 1 return URL and 1 email auto-responder template.
Not in free version. You can in Pro - available here.
If you've tried all the obvious stuff and it's still not working please request support via the forum.