Disclaimer: I'm no economist. This discussion is getting out of my comfort zone, so I skimmed wikipedia. I recommend you do likewise (and maybe browse a library). Here are my two cents anyway.

So is real-estate rent-seeking? How is it different from domain sharking?

In your draft, I think you're confusing distribution with land-development with ownership.

• A Real-Estate Agent facilitates exchange. They create a market by matching buyers to sellers. They are valuable middlemen because buying and selling real estate would be more difficult otherwise.

• A Land Developer takes a rotten piece of land and adds value before passing it on to a buyer; value is added to both parties (assuming a willing buyer exists).

• A Domain Shark is a parasite. They just hog a resource and don't improve it. The Shark is a middleman which benefits himself to the detriment of the rest of society.

The point is that a single person can wear many hats. But for the sake of analysis, we need to recognize distinct responsibilities.

What allows someone to extract rent?

See the bottom of wikipedia. Most seem to fall under either Law or Racketeering.

Are there degrees of rent-seeking?

Value is tough to pin down because it's an implicit, nebulous concept. It's supposed to be the "objective/aggregate" version of utility whereas utility is considered "subjective/personal". But how do you measure value? If economists could just take a ruler and say "yep, commodity X is worth $Y amount of value", value traders would be out of a job. Value isn't actually objective, it's just an average of what everyone likes.

Even harder is tracking the negative externalities. I think the saying goes "everyone hates lawyers, except their own". The idea is that lawyers are good when they represent one's self (who is clearly the hero) , but the net value to society are outweighed by costs of scummy lawyers who fight with guile and cunning on behalf of the villains. It would sure be nice if society could simply eliminate the scummy lawyers. But lawyers aren't paid by society, they're paid by their clients! Therefore, lawyers persist. (This was merely an example. For the record, I prefer to live in a world where lawyers exist.)

Like what if a domain shark is using the domain to host a site that has tetris, and a few of his friends play it?

About tetris. If your site hosts tetris, no one will begrudge you. You at least attempt to provide a service. So long as you can pay for it, the land-development & service will justify the ownership in most people's minds (in a Virtue Ethics sort of way). But if you want to make a hard case for (or against) the tetris website, you'll want to estimate all the pros and cons Utilitarian-style and tally them up. This includes things like "the carbon dioxide emitted by the powerplant which powers your server."

Utility is in the eye of the beholder. But the Domain Shark is as ugly as sin.

How do you know when no value is being created?

You ask who benefits not from the trade, but from the agent's existence. Let's reason counterfactually: what would the market look like if the restaurant were removed from the equation?

1. If the restaurant is profitable and popular, then customers will feel sad over its absence.

2. If the restaurant is unprofitable and unpopular, then removing the restaurant won't make much difference (and free up resources to be allocated elsewhere).

3. If the restaurant redistributes storebought merchandise where it otherwise wouldn't be sold, then customers will feel sad over its absence.

4. If the restaurant simply clears out everything from the Walmart next door, repackages it, and jacks the price, then customers will feel glad at its removal. Good riddance!

Domain Sharking is an example of Case 4. If Domain Sharks ceased to exist, then its customers would buy the website themselves at lower prices and feel glad. Domain Sharks can exist because each URL is unique by design. So if you want a specific URL, you must go through the Domain Shark. I.e. it's a monopoly void of any positive externalities.

Contrast Domain Sharks with Stock Brokers (for lack of a better example). Stock Brokers give you personal advice for a fee. But you also have the choice of forgoing the advice for a smaller fee by trading online. Brokers in this case offer a useful service by buying stocks in bulk and dispensing advice. And if traders don't like that, an alternative exists.

P.S. After reflection, maybe this is where you're getting confused. You understand that value is created when you make stuff people want. But then you look at Domain Sharks and think "People are buying from Domain Sharks. Domain Sharks have stuff people want. Therefore, Domain Sharks must be valuable." The issue is the Domain Shark is a barrier to what people want rather than a portal, but somehow profit from what looks like (if you squint) twiddling their thumbs.

Are there any well accepted examples?

P.S.S. I believe the canonical example is Digital Rights Management. Why go through record labels when artists can upload?

max( Candy / e^Walking )

fudged with a constant of proportionality (pun intended).

I was confused too. Not only by "did he mean video game?", but also "how does he define sales?".

I picked Super Mario Bros and called it a day.