Maintenance for the week of September 22:
· [IN PROGRESS] NA megaservers for maintenance – September 22, 4:00AM EDT (8:00 UTC) - 10:00AM EDT (14:00 UTC)
· [IN PROGRESS] EU megaservers for maintenance – September 22, 8:00 UTC (4:00AM EDT) - 14:00 UTC (10:00AM EDT)
You can only gain access to this information if you actively talk to a guild trader.
So an addon would need to "save your listings to the savedvars" each time and update it for new listings/sold items also.
+ other stuff to do and check.
I guess this isn't very easy doable and I bet it will be within AwesomeGuildStore one day ;-)