Question:

Reuven wants to buy a house from Shimon but doesnt want to pay the price he is asking. He hits on a plan how he can get Reuven to come down in price. He will get a couple of his friends to approach Shimon as potential customers and when they hear the price he demands they will back out, claiming that this house is way overpriced, and thus influence Shimon to lower his price for Reuvens sake. Is this ethical?

This is unethical and forbidden by halacha. Two Talmudic examples are cited as the basis for this ruling.

In Mesechta Succah (34b) we find that the Sage Shmuel was enraged at the price gouging perpetrated by the merchants selling hadassim for use in fulfilling the mitzvah of the Four Species on Succot. He insisted that they lower their prices or else he would issue a ruling that hadassim whose tops were missing were also kosher. Since there were plenty of such hadassim available, the price of even the perfect hadassim would drop.

In his commentary, Ritva notes that Shmuel held that imperfect hadassim qualified for the mitzvah but that there was a preference to use perfect ones. He was therefore not manipulating the price of the perfect hadassim through any falsehood. From this we can deduce that if hadassim without tops were indeed not kosher Shmuel would not have made such a threat. The spurious bargaining of Reuvens friends is therefore forbidden both for being a lie and a deception. In addition, Reuven is guilty of transgressing the Tenth Commandment which forbids one against coveting anothers home to the point where he tries to pressure him to sell at a price to which he is not agreeable.

The flip side of such unethical behavior is found in Mesechta Kiddushin (beginning of Third perek) of the Jerusalem Talmud. There we learn of a strong condemnation made by Rabbi Zeira of a fellow who saw someone about to purchase something and offered the seller a higher price, only in order to force the original buyer to make a higher offer.