Is it permissible to buy shares from an Islamic bank with the intention of selling them immediately to obtain money? Is this considered Tawarruq? And what is the ruling on Tawarruq?

Buying shares is permissible if they are in permissible institutions, not forbidden or dealing with usury. It is permissible to buy shares then sell them for less than the purchase price to obtain money, provided that the buyer is not the first seller (the issue of Tawarruq). As for selling an item back to the first seller for less than its original price, this is not permissible, because it is the forbidden 'Inah sale.