Maintenance for the week of January 12:
• [COMPLETE] ESO Store and Account System for maintenance – January 14, 9:00AM ET (14:00 UTC) - 12:00PM ET (17:00 UTC)
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 ;-)