set?” is a question that needs to be asked before you buy. It only takes one bad box spring or mattress set for the whole set to go down in flames, especially if it was assembled by someone who doesn’t know what they are doing.
It is important to get your bed right when you first move into the house so that there will not be any problems with it in the future. A good rule of thumb is to replace your bed every five years, but this depends on how frequently you use your bed and how many people sleep on it at once. If all things are equal between replacement time periods then I would say replace yours every four years just because of the cost savings aspect. And remember never buy used furniture!