Yes and no. Given the kind of volume Knight purchased during the faulty trades, it could be difficult to offload that many shares on the open market in a timely manner. Maybe those stocks are hot Monday, maybe they're not. Knight needs capital yesterday to keep floating, so they're likely looking to sell everything in one basket.

As for GS's motivation, they're buying at a discount. Due to the time sensitive nature of Knight's predicament, they're probably trading the portfolio to Goldman at a reduced rate. Unlike Knight, Goldman has the cash to sit on it for a while and sell the shares directly out into the open market, even if it takes a few days. Given the discount they bought the shares at, they're likely selling with a decent margin.