"We buy fruit," or "We buy fruits" is the same thing in English. For example, you ask me what I bought at the store today. I say "Fruit". You could then say, "What kind?" So fruit = fruits. Therefore, "We buy fruit" should be accepted. :)
We buy fruits even sounds rather strange, emphasizing as it does that one is buying different kinds of fruit.
I agree that it would be good to accept. We do get insight to the Hindi brain here tho. Swap in "vegetable". In English, we buy vegetableS but in Hindi it would be we buy vegetable (sabzi)!
“We buy fruit.” Fruit is both singular and singular in English as it is a collective noun.