Well... the answer is not a Yes or No. It s deeper.

You can t sell something you re not owning. 

How comes you play on something you re not owning? It s deeper again :-).

Blizzard is selling you the right to play on an account, with an abusive contract you have to agree with to be able to create your account ( yeah, that box you have to click on - I agree - otherwise you re not allowed to create your account or enter the game ). Ofc, the content is owned by Blizz. All the drawings and the art, the story, the characters, everything is owned by them. So it is for sure illegal to start a private server and make money with that. This way you will be using Blizz s property to make profit. They can sue you for that and they for sure will win the case. 

But let s get back to your account. You paid for it. Not for the art or for the right of the content, you paid for the right to play the game. Than you invest money for subscription, and subscription combined with your work ( well, let s call it gameplay ) creates the final product - your main character. The product of your work and money invested into the monthly subscription are legally yours and it is your right, anywhere in this word, to give them / sell them to someone else.

So if you re selling your account, you are not selling the art ( you can t even do that cose they are not yours to sell ), but you are selling the right to play your character.

Here comes another story. Blizzard forbidden that with it s EULA, meaning internal rules, those you Agreed with ( remember ? ) . Keep in mind, EULA is not a low si even if EULA says that, selling your account doesn't become illegal. 

Why are they forbidding the sell. For the reason they are not selling already geared up characters, or items or even gold in-game: for the correct reason they believe that this will scary the customers away. That s pure psychology: IRL you go to work and work hard for some low salary, than you drive back home and some rich guy crosses your way with his big Range Rover, police stops him and he gets away with a small ( for him ) amount of money. You dont want the same thing to happen when you get back home: the same rich guy buys his UBER char from Blizz and kills you instantly with the use of his money. You really want your skill and invested time to matter in-game. That s why Blizz will protect you rather than the rich guy. Because he s 1 and there s thousands like you. But this internal reasoning doesn’t make ( again ) you account buy/sell deal illegal. 

The only thing that Blizz can do, is ban your account. This happens only if you buy, let s say, a R1 account, go for some PVP ( btw, you can t buy the skill, right ? ) and than show-off saying something like “hahaha, 1k eur on ebay and I can kill you all losers  “ . Than those “losers” will report your account, Blizz will track some IP s and, even if they wont have proves, they can simply warn you with a 72h suspend. If that happens again and again they will ban you.

In my Eu-Accounts.com experience, more than 3000 accounts were sold, and Blizzard never did anything agains that. As long as this happens quietly, they know they have no legal strings to counteract that, and they don't even want to, All of this accounts are attracting subscriptions, transfers etc.

So, once again: 

Is it legal to buy and sell WoW Accounts: sure it is!

Does Blizzard quietly agrees with that : sure they are.

is Blizzard banning sold accounts: never in my 8years experience.

Was anyone ever sued for selling or buying accounts: never in written history ( they only sue private servers that doesn’t go off after repeated warnings  ).