Is it permissible to sell shares of an interest-based bank at their current price, or should one wait to recover the principal, and is Zakat obligatory on them?

It is not permissible to buy shares in a usurious bank. Whoever has done so must divest themselves of them by dissolving the contract and recovering the principal. If they are unable to recover it, then do not sell your shares. If profits are distributed and the amount of the unlawful profit becomes clear, then donate it for the benefit of the Muslims. If you do not know the amount, then donate half of it and utilize the rest.