There is a Torah commandment not to desire your friend's possessions.
Definition of Lo Tachmod
- Desiring someone else's possessions is not a transgression of this commandment, per se. The prohibition is to pressure someone into selling you something he did not want to. 
- One who pressures another person to sell him something that he didn't want to sell, is in violation of lo tachmod.
- Pressuring somebody to give you money doesn't technically qualify as lo tachmod.
- These prohibitions (Lo tachmod and lo titaveh) do not technically apply to acquiring property from a non-Jew. Nevertheless, one must be careful in his interactions with a non-Jew to conduct his business fairly and pleasantly.
- There is no prohibition for something that can't be acquired. For example, it isn't lo tachmod to be jealous of someone's wisdom and go and acquire it. However, being jealous of someone else's wife and causing her to get divorced and married to you is lo tachmod.
- It is permitted to negotiate with a buyer to buy an item and offer prices until he is agreeable as long as one doesn't pressure him to sell it.
Pressuring Someone into Giving a Gift
- Pressuring someone into giving you an item that he does not wish to give you is, according to many poskim, a violation of lo tachmod.
- One may ask the owner of an item if he would consider selling it and that is not considered pressuring him. However, once he refuses, it would be problematic to harass him with comments such as, "why don't you sell it to us," or "be a sport," "please change your mind," or the like.
- If the owner expressed no interest in selling, one would not be allowed to raise the offer. But if he indicated that he would consider selling at a higher price or that he would reconsider, it would be permissible to approach him again.
- One is permitted to show the owner why it would be in the owner's best interest to sell, provided that it actually would be in his best interest.
- One may not send a distinguished person to convince the owner to sell, because the owner might not feel comfortable saying no
Definition of Lo Titaveh
- Lo Titaveh is scheming to pressure someone into selling you something. The thought about how to do it, is the transgression.
- Lo Tachmod: Principles and Applications by Rabbi Yona Reiss
- Lo Sachmod by Rabbi Dov Kahan
- Article on Lo Tachmod by Rabbi Josh Flug
- Parashat Yitro, Shemot 20:14 and Parashat Vaetchanan 5:18. Shulchan Aruch CM 359:12 writes that these two are two separate prohibitions: לא תחמוד and לא תתאוה.
- Rambam Gezela 1:9, Sma 359:17, Aruch Hashulchan 359:8. Aruch Hashulchan writes that although this isn't technically the prohibition, it is still a negative character trait that causes great harm. Halachos of Other People's Money pg. 71 agrees.
Smak 19 writes that even jealousy in your heart is prohibited by the Torah, but one is not punished for this until he takes action.
- Halachos of Other People's Money pg. 71-72
- Shulchan Aruch 359:10, Levush 359:10, Aruch Hashulchan 359:9. Shulchan Aruch 359:9 writes that such a person is disqualified from testifying on a rabbinic level. Sma 13 writes that although you are only disqualified on a rabbinic level, you have still violated the Torah prohibition of Lo Tachmod as is explicit in Shulchan Aurch 359:10.
- Halachos of Other People's Money pg. 72 note 198 based on Rav Yerucham Fishel Perlow in his commentary to the Sefer Hamitzvot of Rasag (pg. 337) and Rav Yisrael Yaakov Fisher (Even Yisrael 8:105: "Kol Zeh"), Rav Zalman Nechemia Goldberg (Kovetz Beit Aharon ViYisrael Year 14: Gilyon 4. Rav Fisher writes that there is no lo tachmod on something you can't see such as money. It only applies to objects. The Chafetz Chaim (Kitzur Hamitzvot Lav 1) writes that it is lo tachmod for a perspective son-in-law to pressure his perspective father-in-law to give him a larger dowry because of lo tachmod. However, Rav Fisher explains that he was referring to where the son-in-law was desiring specific objects.
- Halachos of Other People's Money pg. 74, Minchat Shmuel CM 4:12
- Aruch Hashulchan 359:10
- Sama 359:19
- Minchat Shmuel CM 4:12 based on Ben Ish Chai Ki Tavo 17, Erech Hashulchan 359:8, and Betzel Chachma 3:43.
- Halachos of Other People's Money pg. 73 based on Shaare Teshuva of Rabbenu Yonah 3:43, Sefer Hamitzvot Hakatzar of the Chafetz Chaim Lo Taaseh 40. see also Shu"t Bitzel Hachochma 3:43 who quotes opinions on both sides.
Sefer Hamitzvot Hakatzar of the Chafetz Chaim Lo Taaseh 40 gives the example of a chattan who pressures his prospective father-in-law to give him something that wasn't initially included in their agreement, the tenaim.
- Halachos of Other People's Money pg. 74. see however Shaare Teshuva of Rabbenu Yonah 3:43 who warns that if you are a distinguished person who the owner cannot refuse, you shouldn't even ask unless you are certain that he would be selling it to you wholeheartedly.
- Halachos of Other People's Money pg. 74-75 in the name of Rav Elyashiv
- Halachos of Other People's Money pg. 75
- Halachos of Other People's Money pg. 75
- Shulchan Aruch 359:10, Halachos of Other People's Money pg. 72