Counter offers are an available method to allow some back and forth between a seller and a potential buyer. Say a user has sent you an offer for an individual asset that you own (NOTE: This is not possible for Collection offers) and the offer is lower than you would like for the asset. You can then send a counter offer to the user with an amount you would be willing to accept. The user will receive a notification and then has the option to modify their offer or cancel their offer. 

NOTE: If they modify the offer to match your counter offer, you still need to go in and accept the new offer.