The sell at guild store icon is only applyable to items which are not bound to your account/character nor stolen!
Stolen and bound items cannot be sold at a guild store so it makes no sense to mark them with that dedicated marker icon.

But you are able skip the "is item bound" check via an exclusion setting at the marker icons -> sell at guildstore submenu.
The "is item stolen" check cannot be excluded though!

Same applies to the "sell at guildstore" keybind.