helo im alex gafin i really wannna learn chinese from you.

I give you an example from dictionary, then for that example I believe you can understand the reason. Example sentence from dictionary: "We were seated in the first row of the theater by the usher" So you can see that if "we were seated by someone". That means we sat down there following someone's direction, in this case - usher. Now we go back to your sentence, in this case sales take place outside the hall, so I assume that audience were “seated” on the benches “under someone’s direction”, in this case maybe salesman, maybe the host of the event. Because it is under someone’s direction, seated is the one to use. I hope it helps~