What Natural Resource Is Easier to Get on Earth Than Nearly Anywhere Else?

In the course of a book review, Eric S. Raymond pointed out that there isn't much need for extraterrestrials to grab natural resources:

Yet another is that Mays has managed to invent a reason for aliens to go to the colossal expense of invading Earth with sub-lightspeed starships that is both novel and plausible-seeming.

This is harder than it sounds. Want metals? Mine your own asteroid belt. Want volatiles? Scoop ‘em off comets or an ice moon or somewhere without a deep gravity well and obstreperous natives. Want energy? You have easy access to your home sun. Want slaves? Build robots. You can’t want our women, the orifices won’t fit and the pheromones are all wrong.

A natural resource that's easier to get on Earth than nearly anywhere else? Chlorine. As far as I know, it's more concentrated in Earth's oceans than anywhere else in the Solar System.

On the other hand, they probably can get to Earth-like worlds without natives and, if they're passing by and have to tank up, they can probably buy the trace amounts needed. “Send us 1 million tons of salt, and we'll give you the blueprints for a working nuclear fusion reactor.”