"They are buying small ships."
Translation:Küçük gemiler alıyorlar.
"Küçük gemi satın alıyorlar" They are buying a small ship.
"Küçük bir gemi alıyorlar." They are buying a small ship.
I cannot verify the more common use of, "almak" or "satın almak" & I'm sure your answer is correct without using, "bir."
Duo accepts, "buying" without using, "satın" so I think it's optional.
You have created a brand new question & answer.