Maintenance for the week of November 18:
• PC/Mac: No maintenance – November 18
• ESO Store and Account System for maintenance – November 19, 9:00AM EST (14:00 UTC) - 6:00PM EST (23:00 UTC)
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There are only few items that you could consider "permanent". The deciding factor will be demand. If people continue to buy them, they stay. Once every who wants them already has them, they go.
The ones with a short store life tend to be the "special offer" ones that need the slight demand boosted by exclusivity.