I'm re-posting this official Zenimax post from several months ago since people are still getting burnt by buying their ESO key from less than trustworthy sites. The list below is a complete list of official retailers from ZOS

We’ve recently seen an increase in the sale of fraudulently obtained digital game keys for The Elder Scrolls Online: Tamriel Unlimited.

We want to remind gamers to shop wisely and only purchase online keys from a reputable source. Fraudulently obtained keys obtained via Steam and then re-sold via third-party websites is a violation of both our and Steam’s Terms of Service. We will be deactivating all game accounts created with such stolen keys starting on Tuesday, May 26th. Affected users will receive an email with instructions on how to regain access to their game account via a valid game purchase.

The companies selling stolen keys make a practice of using stolen credit cards, or misappropriating credit card information from their own customers to buy codes from legitimate retailers. Often these companies have the lowest price available anywhere for the digital item they are selling. Customers who purchase from these vendors are at increased risk for identity theft.

Extreme discounting below the published prices on our website may be a clue that the digital key purchased for the game may have been fraudulently obtained.

We want to ensure that you have the best game experience possible. To avoid unknowingly purchasing a stolen game key we suggest that you purchase from our site or one of the official retailers listed below.

Global

• 2game
• Allbrary
• AtGames (Direct2Drive)
• DLGamer
• Gamebillet
• GameFan Shop
• Gamesrepublic
• Gamesrocket
• Get Games Direct
• Green Man Gaming
• Humble Store
• Metaboli / Gamesplanet
• Microsoft
• Nuuvem
• Paddle
• SaveMi
• Sila Games
• Smilegate
• Sony
• Steam
• The Licensing Agency
• Tradebyte
• Tranquiltechnology
• Ubisoft Uplay
• WinGameStore
• Xanatan
• Also Digital BV
• Exertis Ztorm
• Game Retail Stores ( Game Digital Limited )
• Gamersgate
• Nexway
• Soft Distribution Gmbh

North America

• Amazon
• Best Buy
• Busca Corp
• Buy Egames
• GameStop
• Imperial Games

Europe

• 1 Tonne de jeux
• 1c Digital / Muve/softclub.ru
• ALSO Shops
• Alternate
• Alza
• Au fond à gauche
• Boonty
• Boulanger
• CDiscount FR
• CDON
• Cenega Digital – Muve
• Central Point NL
• Codora
• Computersalg
• Darty
• Digital Virgo
• Dream Game
• Elkjøp AS (Dixons)
• Entertainment Trading
• epay
• Fnac
• Foniks
• Game Sultan
• GAME.co
• Gamedigi
• Games Republic
• Games Rocket
• Gamesload
• Gry-online
• IGE Group
• Intexmedia ES
• Jeuxvideo.com
• Jimms
• JRC
• Keto Media GmbH DE
• Kioskea
• Komplett
• konsoleigry
• Micromania
• More Than Games
• Multiplayer IT
• Muve
• Notebooksbilliger
• Orange
• PC Garage
• Playcycle
• PlayGames
• PS
• Rue du commerce
• SFR
• ShopTo
• Smadis
• Softonic
• Softwareload
• Spaceworld
• Starzik
• Tech Data
• Tones
• Toomai
• TTNet
• UnlimitedGameStore
• ZokkusMacGames_FR

Africa

• BT Games

Hong Kong

• 520 Game Limited
• Alta Multimedia Limited
• BE Advance Computer Systems Co.
• Black Hand Fu
• CC Game Company Limited
• Chinesegamer International Corporation
• Gallop Computer Technology Limited
• GameNeed (HK) Limited
• MSI (Micro-Star International)
• Msystem.com.hk Limited
• Sino Legend International Company
• Six PC Company
• Success Comic Book & Network Game Gallery
• Tung's Computer Technology Co.
• West City Computer Multimedia Ltd.

Oceania

• Savemi
• Didgio Pty Ltd

Russia

• 1c Publishing / 1c – Soft-Club

South America

• B2C Express
• GatoSabido
• Incomp
• Nuuvem
• Paodeazucar
• Playspot
• Saraiva
• Starmedia

Southeast Asia/Japan

• Friendster.com
• GamePia
• H2 Interactive
• StraStar
• Tact Interactive Pte Ltd
• Beijing CE-Asia Co Ltd
• One Station Technology Co ( Gamesource )
• Transcrane Enterprise Co Ltd ( T/A Kanezo )
• SquareEnix e-store