The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Ki Tavo (17), discusses the prohibition of "Lo Tahmod" – "You shall not covet" (listen to audio recording for precise citation). He writes that one violates this prohibition by applying pressure upon his fellow to sell him his possession, such as his home or utensils. If a person desires his friend’s property, which the friend does not wish to sell, and he pressures the individual or sends people to pressure him to sell it, and the friend ultimately relents, then the buyer has violated the prohibition of "Lo Tahmod." Even though the friend ultimately agrees to the transaction, and the buyer pays a fair price, he has violated this transgression because he applied pressure to coerce his friend to sell something he had not intended to sell. The Ben Ish Hai notes that one violates this prohibition only when the owner finally relents and proceeds with the sale.
There is a separate prohibition, the Ben Ish Hai writes, of "Lo Titaveh" (literally, "You shall not desire"), which refers to the feeling in one’s heart. A person violates "Lo Titaveh" when he begins devising a scheme to persuade his fellow to sell him the item in question. Already at that moment, one transgresses this prohibition because he begins thinking in his mind of ways to obtain the coveted object. The Ben Ish Hai emphasizes that one does not violate this prohibition if he simply wishes in his heart that the owner would agree to sell the object. The transgression is committed only when one begins devising practical strategies for convincing the owner to sell.
The Ben Ish Hai’s ruling follows the view of the Rambam, who held that one transgresses "Lo Tahmod" once the owner acquiesces and agrees, under duress, to sell. The Ra’abad (Rabbi Abraham Ben David of Posquieres, 1120-1198), in his critique of the Rambam’s Mishneh Torah, disagrees. He maintains that to the contrary, if the owner consents to the transaction, then the buyer cannot be said to have done anything wrong. Even if the owner agrees due to pressure, the Ra’abad argues, a buyer who purchases an item with the owner’s consent does not commit any offense. Halacha, however, follows the Rambam’s position, that one indeed transgresses this prohibition by pressuring the owner to the point where he relents and agrees to sell the item which he had initially not intended to sell.
Summary: The Torah forbids pressuring somebody to sell a possession which he had not intended to sell. A person violates one prohibition the moment he begins devising a scheme to pressure the owner, and he violates another prohibition if the owner ultimately relents and agrees to sell the possession.