How do I reset Drupal Commerce orders, profiles, carts etc?
I don't recommend truncating database tables, because you also have to consider field data / revision tables - that can get nice and tricky if you've used the same field on entities that you want to delete and on those you want to keep. There also may be other contributed modules on the site that have stored data related to these entities. Without using the API to delete these entities properly, you'll end up with this stale data laying around that may or may not cause problems later.

If I had to do this I'd either do as the other answer recommended - go through each list and use the delete links - or in the case of large data sets, I'd add Views Bulk Operations to the Views so I could delete them en masse. VBO is one of Drupal's best kept secrets - great for automating routine tasks like this.

When deleting Commerce entities, I'd start with payment transactions then orders then products and customer profiles. You need to delete both complete orders and shopping cart orders, otherwise referenced products / customer profiles will not be deletable.