This icon indicates that an item, monster, area, etc. was previously obtainable or findable in-game, but is no longer so. Rare items may be used just like regular items by those who have them, even after they are no longer available to the rest of the community. However, if they are sold, there is no way to get them back.
This icon indicates that an item is not available all the time, but is available on an annual or recurring basis, likely as part of an seasonal event. Once sold, these items cannot be obtained again until the event that grants them returns.
This icon indicates that an item could only obtained by donating to the Kickstarter campaign that helped fund AQ3D! They are now rares to all those who did not donate to Kickstarter while it was active (Nov 20, 2015 - Jan 1, 2016). Backers may obtain these items from the Kickstarter Shop, or by completing quests.