Otzar Beit Din Lechatchila

Is it permissible to buy vegetables from Otzar Beit Din during the Shemitah year without any concerns?

There are kashrut committees that take from Otzar Beit Din, see details in the source, but there are those who refrain from it.

Vegetables that began growing in the sixth year and were harvested in the seventh do not fall under the prohibition of sefichin (according to the opinions of the Rash and Ramban), but they do have the sanctity of the seventh year. Some kashrut committees market these vegetables under their supervision. To avoid the prohibition of commerce and harvesting in large quantities, this is done through Otzar Beit Din, meaning all vegetables are declared ownerless, and the Beit Din takes them under its control, hiring workers to perform the permissible actions during Shemitah. The payment for purchasing from Otzar Beit Din covers the Beit Din's expenses in handling the vegetables, not the vegetables themselves. However, there are kashrut committees that do not sell these vegetables because, according to Rambam, all vegetables harvested in the seventh year are prohibited due to sefichin, even though the halacha follows the opinion of Ramban and Rash that there is no prohibition of sefichin (Chazon Ish, Shevi'it, siman 9, section 17). Nevertheless, they are stringent to consider Rambam's opinion and only sell vegetables harvested in the sixth year. There are also those who believe there is no permission for Otzar Beit Din at all since Rambam did not mention it, although the halacha follows the opinion of Ramban (Vayikra 25) and Rash (Shevi'it, chapter 9, mishnah 8, who ruled according to the Tosefta, Shevi'it, chapter 8), and therefore some Shemitah committees do not sell vegetables from Otzar Beit Din at all. Azmera Leshimcha (Shemitah, issue 2).