I am from Washington State. In the absence of agency disclosure to the contrary, all buyers are legally represented by the agent they are dealing with, who, by virtue of representation, then becomes the selling agent.
As a side note, if a purchaser of property happens to be an agent, it would be highly unlikely that he or she would hold himself or herself out as representing the seller, which, by virtue of common sense would make little sense.
As to commission earnings, the selling agent (Purchaser) would be entitled to selling side. A licensed agent, though, must disclose that he or she is a licensed agent. To represent yourself is not illegal.