is it muttar to sell one’s private sefarim?
1) just for the money
2) to buy other sefarim he would rather have
It it permitted to sell one’s sefarim for the money, but the practice should be avoided where possible. It is certainly permitted to sell personal sefarim for the purpose of buying other sefarim.
Sources: See Beis Yosef, Choshen Mishpat 248, who writes that there is a distinction between a sefer Torah and other sefarim, and adds that even according to Rosh who writes that today sefarim are like a sefer Torah, they are nontheless not as severe. In fact, the Rosh who equates sefarim to a sefer Torah differentiates between public and private sefarim, stating that the prohibition applies only to public sefarim (see Tur, OC 153 and YD 282). Beis Yosef suggests that according to Rosh, a private individual who sells his sefarim does not see siman berachah from the income, referring of course to financial blessing, and not to gains of learning when a book is sold to buy another. Rambam does not mention this distinction, and Shulchan Aruch brings both opinions, yet Rambam does not equate regular sefarim to a sefer Torah. See also Achiezer, 3:79, who writes that one does not make a mechaah against one who sells his sefarim. However, see Rabbeinu Yonah on the Mishnah “Buy truth and do not sell it,” who applies the teaching to the sale of sefarim. Again it seems clear that the advice does not apply when sefarim are sold for the sake of furthering one’s learning by purchasing other sefarim. See also Chasam Sofer on Orach Chaim 153, who makes a distinction between selling a single copy of a book and selling a double.