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I don't have any experience with the epos driver, but this looks like a fairly standard permissions problem.

The general things to do are:

Examine the permissions on /dev/bus/usb/003/007 with ls -l /dev/bus/usb/003/007
Manually grant permissions to all users for this device: sudo chmod 777 /dev/bus/usb/003/007
Confirm that the permissions are now set to read/write for all users: ls -l /dev/bus/usb/003/007
Try your node again now that the permissions are less restrictive: rosrun epos_hardware list_devices

If those steps work, then you've confirmed that the permissions are the issue, and you can work on setting up a udev rule to set the permissions correctly in the future.

If those steps don't work, update your question with the output from each command, and I'll help you troubleshoot further.

I don't have any experience with the epos driver, but this looks like a fairly standard permissions problem.

The general things to do are:

Examine the permissions on /dev/bus/usb/003/007 with ls -l /dev/bus/usb/003/007
Manually grant permissions to all users for this device: sudo chmod 777 /dev/bus/usb/003/007
Confirm that the permissions are now set to read/write for all users: ls -l /dev/bus/usb/003/007
Try your node again now that the permissions are less restrictive: rosrun epos_hardware list_devices

If those steps work, then you've confirmed that the permissions are the issue, and you can work on setting up a udev rule to set the permissions correctly in the future.

If those steps don't work, update your question with the output from each command, and I'll help you troubleshoot further.

UPDATE

I suspect that the udev rules you attempted to set up on your first attempt were correct, but they weren't loaded properly until you rebooted; hence why your permissions issue is "magically" gone.

I can't help you with the epos driver itself (no experience there), but the errors suggest that you should carefully check that the names in all of your config files match.