Books were not selling?

[mallu]

Is this sentence correct
He stopped writing since his books were not selling

[Corybobory]

It sounds ok to me!

[Daniel]

It's a so-called "middle voice" verb, common in English with no special morphological marking.

That sentence is a little colloquial but perfectly normal.

[mallu]

Could this be corrected as "He stopped writing since his books were not being sold? I mean to make it look more formal.
Thanx in a dvance

[ibarrere]

It sounds more natural to me as "he stopped writing since his books were not selling" but either way is grammatical.

[lx]

For me, there is a (pretty clear) difference between those two sentences.

My books were not selling : The public / target audience were bot buying my books
My books were not being sold : No commercial outlet would put my books on the shelves

Perhaps the explanatory sentence is a little old fashioned in the age of Amazon, but the point is still clear.

[Daniel]

Agreed. The second one is ambiguous but that (other) meaning is most salient.

A natural paraphrase would involve a subject like "no one was buying the books."

[ibarrere]

For me, there is a (pretty clear) difference between those two sentences.

Agreed. After I posted I was trying to sort out the semantic differences between them. The "books were not selling" definitely implies that the public wasn't into them. The "books were not being sold" likewise implies that some persons unknown were responsible for the state.

[mallu]

Then,the sentence "his books did not sell" also is O.K. Isn't it?

[lx]

Then,the sentence "his books did not sell" also is O.K. Isn't it?

Yes