Both your website and your widget must be able to answer these questions within 30 seconds for the most ignorant, distracted user with 3 children trying to climb into their lap.
This user might have never bought online before, and if they have - they have no idea what the backend order process looks like.
Answering these questions here is only a beginning. If _anyone here_ cannot get your site before getting frustrated, you haven't explained your system well enough yet.
Remember, the majority of users (by a huge percentage) are going to see your widget first. Your widget is going to advertise your solution to potential sellers who see it on someone else's website.
If _any customer_ gets frustrated at your widget, it is going to get pulled from sites.