Reading the Rambam Hilchot G'zela V'aveda Halacha 9-10:
Anyone who covets a servant, a maidservant, a house or utensils that belong to a colleague, or any other article that he can purchase from him and pressures him with friends and requests until he agrees to sell it to him, violates a negative commandment,even though he pays much money for it, as Exodus 20:14 states: "Do not covet."
The violation of this commandment is not punished by lashes, because it does not involve a deed. One does not violate this commandment until one actually takes the article he covets, as reflected by Deuteronomy 7:25: "Do not covet the gold and silver on these statues and take it for yourself." Implied is that the Hebrew tachmod refers to coveting accompanied by a deed.
Anyone who desires a home, a wife, utensils, or anything else belonging to a colleague that he can acquire from him, violates a negative commandment at the time he thinks in his heart, "How is it possible to acquire this from him?" and his heart is aroused by the matter, as Deuteronomy 5:18 states: "Do not desire...." Desire refers to feelings in the heart alone.
I guess that to violate coveting you need to do tricks to get the item that belongs to another Jew (but if you directly offer to buy it you are not in violation).
But to violate desire you the thoughts of tricks are not necessary to be in violation.
Please correct my understanding.
What are the exception of desiring and coveting another Jews property, for example practically, if you see something by your friend can you want it (is it not possible that he wants to sell it)?
Or is the buyer never allowed to offer to buy something until it is offered by the buyer for sale?