Musty smells are some of the most unpleasant. Check under your sinks for any drips or leaks and around toilets from condensation drips or leaks around the base.
Laundry rooms often smell musty and, in most cases, this is to go with dirty clothes hampers and baskets. I'll never forget the multi-million dollar listing with the smell of dirty clothes in the master closet. It's disgusting and turns buyers away. Remove laundry and waste baskets that cannot be sanitized and, if you must, keep dirty clothing in a sealed hamper.
I like using dehydrating products like Damp Rid in closets. Closets on exterior walls can get musty especially those on north facing exterior walls. Keeping those areas very dry is key to keeping them odor free.
Basements with finished rooms that are occupied by people should NEVER smell musty or feel damp. If they do, work needs to be done to correct the problem and there are amazing products available to accomplish this. Think about how many people live in houseboats and nearly all of them have below waterline living space. Costs for making repairs vary greatly depending on what needs to the done. Regardless of cost, home values are tied to the amount of livable space.
When selling, it is best to correct moisture issues before going on the market. If the issues are too far beyond the scope of the seller to correct, the buyers will have to calculate those costs into their offer. Seller cannot expect to get the full value of any square footage that is not fully livable.