다는 거 보다

judging from the fact that X

Takes a reported fact, treats it as evidence, and draws a conclusion from it: 내년까지 완판됐다는 거 보니까, 'seeing that it's sold out through next year...'. It appears almost entirely as 보면, 보니까 or 봤을 때, with the inference following. What precedes is something heard or read, not something witnessed — for that you would hear -는 걸 보니까 about a scene in front of you.