卖座 is a stative verb (s.v.) that describes movies, plays, shows, or performances that are commercially successful — that is, they 'sell seats' and attract a large audience. It is the opposite of 冷门 (unpopular). You use it with 很 or 非常 to say something is a big hit, or with 不 to say it flopped. The term applies specifically to box-office or ticket-sale performance, not to general popularity like 受欢迎.