are often wanting you to do things and say things that will improve the time to sell the home. Sometimes they do not act in the favor of the home owner.
Contact another realitor for a second opinion. Do some behind the scenes questioning of this statement.
We know that something is wrong somewhere for the basement to leak. It would be nice to find out where it is coming in from. You also have to declare that the basement does leak during heavy rains. Disclosure Law.
You can do two things, fix it yourself, or cut the asking price to accomodate the seldom leaking basement.
What will kill you is that if you have a group of people looking at the home and you are in a heavy rain at that time, and they see the water in the basement, people will think that the basement just plain leaks, not only during a heavy rain but always.
Your plans for the basement and the future owners plans may be different. They may want a living area down there. So, I would fix it.