In the superficially similar cases of many Banksy works across London, the answer is you chisel out entire sections of buildings and sell them to private collectors. eg:
...wondered if Banksy would get a mention. Also of note is the occasional "Oops it got painted over / destroyed" ending to his art which nature can also replicate. Might take eons, but the planet doesn't run on my clock.
An interesting introduction but the article unfortunately falls short without really answering its title question. IMO it could have continued to discussing the same dilemma faced by most conceptual art. Also surprising there is not a single mention of the contemporary Andy Goldsworthy [0], who has done a great deal to promote environmental art.
Is there really an interesting question here, though? The art world has for years had no difficulty trading in immaterial works of art, or, for example the right to exhibit a performance. In the case of something simple that anyone could reproduce, like a very simple performed gesture, what actually gets sold is the right to assert that a given performance is actually an authentic, canonical instance of the work.
I've never encountered any interesting nuances or complexity when it comes to the sale of conceptual work. It is just very straightforward.
An additional article [0] showing some of the work [1] made me smile thinking about some future civilization finding these and trying to figure out why we built them or what they were used for.
Those interested in this sort of thing may enjoy the film series on sculpture, "The Sculpture Diaries" by Waldemar Januszczak, in particular, the 3rd film on Land Art. I was unable to locate a streaming source for the film, but trailers are still available [1], [2].
Annoyed that they never directly addressed the answer. But it seems obvious that it is by sale of the land, or rights/access to it.
As anyone would agree Architecture is art, this question has long been settled. Essentially clickbait for an article interesting enough not to need it.
http://news.bbc.co.uk/1/hi/entertainment/7188387.stm
http://www.bbc.co.uk/news/uk-england-gloucestershire-3714847...
http://www.bbc.co.uk/news/uk-england-london-23461396
http://www.bbc.co.uk/news/uk-england-kent-30242249