Maintenance for the week of December 16:
• PC/Mac: No maintenance – December 16
• NA megaservers for patch maintenance – December 17, 4:00AM EST (9:00 UTC) - 12:00PM EST (17:00 UTC)
• EU megaservers for patch maintenance – December 17, 9:00 UTC (4:00AM EST) - 17:00 UTC (12:00PM EST)
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 ;-)