KNOWING someone is not a disclosable "relationship".
You had to have signed the form for her to represent you both in a Dual Agency Agreement.
You had the chance to read all the documents, include that you were paying for the Warranty:
You cannot blame these things on her!
You don't mention how much the selling price was, but I can imagine.
You have sold the house, which was your objective, and now you taste sour grapes.
Get over it, and move on.
If the listing agent had a relationship with the buyer they should have disclosed that to you. You need to consult a real estate attorney to find out what your rights are, but there are things you can do without taking legal action, such as filing a complaint with the DRE and/or their local MLS. That said, proving these things is very difficult, especially in a he said/she said situation such as this.
Another thing you can do is post a review of your experience on yelp and other similar sites, but make sure to be factual.
If you didn't get any offers when your home was listed for sale, even when you lowered the price, then assuming the home was listed for sale on the MLS and that your agent marketed it for sale online etc., the likelihood is that it was overpriced. The Lafayette market has been strong for a while now and well-presented, properly marketed homes have been selling quickly, provided they were not over-priced.
Having said all that, I believe your agent should have disclosed that the buyers were friends, if that was the case. In addition, when I list homes for sale, I won't represent both sides of the transaction as I believe it reduces the level of service I can provide to my seller. It surprises me that more sellers do not seem to care about that.
J Rockcliff Realtors
I believe the relationship should have been disclosed - particularly if they are friends - but if the house was listed in the MLS, and held open, it probably got fair exposure.
A real estate attorney would be the next step - with a short consult they can likely help you figure out if there is a further case.