A seller client would like an open house this weekend. The seller's agent is not available and the seller decides s/he will host it instead. Is this permissible? As the owner of the property, the seller is exempt from the Real Estate Licensing and Registration Act. Just as a seller may sell their own home without having a license, so too, a seller may perform other duties associated with selling the home. The more appropriate question should be, "Is it advisable?".
The seller likely only sees the opportunity to bring more potential buyers to the house. In their excitement at this prospect, they fail to see that they are giving up the benefits of having a REALTOR representative. While this situation may be temporary - only for the duration of the open house - the damage could be costly to future negotiations. The same reasons that a listing agent encourages a seller to use a REALTOR in the first place can be applied to the seller conducting any part of the transaction on their own. Promoting and protecting the best interests of the client often involves protecting them from themselves by helping them understand the pros and cons of a given situation.