Overall: You can only send to sendable data extensions.
To achieve what you are trying to do, you would need one of two things:
1) Either create a data relationship inside email studio (subscribers => data relationship), and then create a filtered data extension based on "past users" who have a record in the non-sendable DE (e.g. customer shop info != null), or create a boolean field with default value = true, and filter on that
2) Use SQL to generate the audience you need in a new sendable DE. (it can be combined with automation studio and file drops, if it is a process you need to repeat over time).
Linking the data via Contact Builder, like you've done, allows for decision splits inside a journey. That's the primary use case for non-sendable DE's to be linked, to my knowledge :)