I just met a sentence, that is
If someone bought it already, then that item will not show up.
It obviously refers to future, but why is bought used instead of buy?
I consider this same as AFTER, and After you took/after you take? says "You will do this, after you do that,".
I am so confused about this two. Both refer to the future, why is one in the past and the other is in the present?