"He only buys fresh carrots."
Translation:הוא קונֶה רק גזרים טריים.
You can, but the meaning is different. Maybe with intonation and context it can be understood to mean the same thing, but without them, רק modifies the element immediately after it, so this means "he does nothing but buy fresh carrots", or "he only buys fresh carrots, he doesn't eat/sell them" for example. Also, colloquially, you can use רק to say something like, in a very short time, for example: wait a minute, הוא רק קונה גזרים טריים and he will be with you.