Translation:Only the market is of interest to him.
"He is only interested in the market" should be accepted.
"He only takes interest in the market" should also be accepted.
I agree regarding the message, but your version does some grammatical stuff (passive to active voice, change of subject) that Duo might not like.
We're encouraged to do that with the verb gustar. No consistency in this program.
in English this use of passive voice sounds strange - either antiquated or poorly translated