Who do you think those shares are being sold to? Robinhood's brokerage side is restricting trades, but their exchange (the "shadow" exchange they use to make money without charging commissions) is still allowing buy orders, or else they have effectively suspended GME on their exchange and have to route the sell orders elsewhere. Again, it is not possible for an exchange to only allow selling of a stock, because every share sold is a share someone else is buying.