ANSWER: In most states, it is the seller, but obligations to disclose information about a property vary.
Under the strictest laws, you and your agent, if you have one, are required to disclose all facts materially affecting the value or desirability of the property which are known or accessible only to you.
This might include: homeowners association dues; whether or not work done on the house meets local building codes and permits requirements; the presence of any neighborhood nuisances or noises which a prospective buyer might not notice, such as a dog that barks every night or poor TV reception; any death within three years on the property; and any restrictions on the use of the property, such as zoning ordinances or association rules.
It is wise to check your state’s disclosure rules prior to a home purchase.