Dual agency is a slippery slope, and the thing you have to remember is that just because the agent is representing both parties, it doesn’t mean that both parties are privvy to all information on the purchase. Buyers do not get copies of sellers' appraisals, just as the sellers don’t get copies of buyers. Fact that there is only one agent involved in the transaction doesn’t change this. The agent’s primary fiduciary duty is also to the seller first, assuming she had the listing before she represented you. Answer to your question is yes, it’s legal.