Maintenance for the week of April 22:
• [COMPLETE] Xbox: NA and EU megaservers for patch maintenance – April 24, 6:00AM EDT (10:00 UTC) - 12:00PM EDT (16:00 UTC)
• [COMPLETE] PlayStation®: NA and EU megaservers for patch maintenance – April 24, 6:00AM EDT (10:00 UTC) - 12:00PM EDT (16:00 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 ;-)