Over the years I have heard that one is not supposed to sell their first, or only, home in Israel. What is the source of this idea? Is it Halacha? Can one sell in order to buy another? Thank you.

Shalom. There is a positive mitzvah to buy a house or land in the Land of Israel (Bamidbar 33, 53) and it's so important that one can even ask a gentile to write a contract on Shabbat, in order to fulfill the mitzvah (Shulchan Aruch Or. Ch. 306, 11). Accordingly, one shouldn't sell their property in Eretz Yisrael unless they intend to buy other property here, or unless their economic situation forces them to do so, and that's the only way they can stay in Israel (the minimal aspect of Yishuv Eretz Yisrael is to live here, even if you don't own property) or in order to start a business in Israel which is also Yishuv Eretz Yisrael. In general, one must be willing to sacrifice up to 10-20% of their possessions in order to fulfill most positive commandments, but the poskim cite that special mitzvot, like learning Torah or to live in Israel, obligate one to give up everything they own, as long as they don't have to beg for a living. For more details, see my article from Tchumin at: http://www.yeshiva.co/midrash/shiur.asp?id=20192 Accordingly, there's no problem to temporarily sell property in Israel (it doesn't matter of it was your first house or not), as long as it's not to a gentile, and it's for the sake of settling the Land of Israel. With Love of Israel, Rav Ari Shvat