If you sell any device which uses GPL-licensed software, you are required to make the software available in the form the devices uses it. The same principle applies to AGPL software - with the added requirement that the software is also made available when the users interact with the software via a network interface, thus the software works server-side.
As such, if you use GPL or AGPL software on a machine, you are required to make an offer to provide the full sources to the code on that device. It doesn't matter wheter GUI or backend. The simple fact is: you convey the software, thus you are bound by the license:
$4: You may convey verbatim copies of the Program's source code as you
receive it, in any medium, provided that you conspicuously and
appropriately publish on each copy an appropriate copyright notice (...) and give all recipients a copy of this License along with the Program
§5 You may convey a work based on the Program, or the modifications to
produce it from the Program, in the form of source code under the
terms of section 4, provided that (...)
§6: You may convey a covered work in object code form under the terms of
sections 4 and 5, provided that you also convey the machine-readable
Corresponding Source under the terms of this License: (...)
And if the software is AGPL, you are required to publish it also, if the software runs anywhere, on that device or a remote server and is used for the functionality of the service in any way. And it doesn't matter wheter it's GUI or backend.
Customers who buy the vending machine fall under both GPL and AGPL.
But your question seems about users of the vending machines who just interact with it to purchase something... tricky. I still feel that an offer needs to be made for the AGPL case - the vending machine is in that case the client which interacts with the server - and then the offer has to be made and the user has to be made aware of the fact; it's just an uncommon, custom client, but that does not mean the AGPL license has no effect - it works just the same.
Quote from AGPL license:
- Remote Network Interaction; Use with the GNU General Public License.
Notwithstanding any other provision of this License, if you modify the
Program, your modified version must prominently offer all users
interacting with it remotely through a computer network (...)