First I would ask your attorney and review my purchase and sales agreement and follow his recommendation and or what is spelled out in the p&s.
Many buyers that plan on flipping homes, will put a clause in that allows them access and the ability to market the home prior to taking possession.
None of this could not be done if it was not in the purchase and sales agreement that you signed.
If they did not have your approval, I would first call your agent to discuss it with the buyers agent and asked them to remove the items from your home and remove the listing from MLS. If this does not work, I would then, contact the agents office manager.
Lastly I would have your attorney do what the law allows him to do, to protect your rights under the p&s and law, because they could be trespassing.
Contacting the board of real estate is more out of your frustration to get even, although the board has no jurisdiction unless he is breaking the law, which if he is, you will get much more satisfaction using your attorney.