The reason why you can't get the achievement from Arcane or Mythic items by equipping

[Samhayne]

I've had a couple people raise the concern that they cannot obtain the Arcane or Mythic Achievement by equipping the item. As it works currently, you need to actually loot the item to receive the achievement. If you purchase it off the Consignment Shop, for example, you will get the item, but you won't get the achievement.

The main reason for this is to prevent scamming. For example:

• You go into town with your Arcane Weapons equipped.
• You get a bunch of tells from people who want you to trade them to you so they can get the Achievement and they promise that they will give them back.

or...

• Your guild member, who you have played with a bunch, asks you to loan him your Arcane Weapon so he can get the Achievement.
• You figure, ok, sure, he is my good guild buddy, right? So you trade them to him. He leaves the guild and logs off. Changes his name and you never see him again.

It is for issues such as the above that the Achievements for Arcane and Mythic items are only given out when the item is first obtained (looted) and not given out for actually equipping them.