Saving Seats
We've all been asked on occasion to "save a seat" for another person, or perhaps we have occasionally offered to do so on our own initiative.[1] It's usually for the sake of getting a "good" seat on a bus or at some type of show and maybe even in the classroom in order to ensure a friendly neighbor at one's side. Saving a seat requires one to carefully guard a seemingly available space from competing seat seekers, some of whom often have their own seat swiping tactics. The more the seats fill up, the more uncomfortable it becomes to continue to save the coveted seat.
Although saving a seat for another person may appear to be an innocent gesture of goodwill, it may actually be forbidden. This is because the Talmudic principle of "zochin l'adam shelo b'fanav" - that one may act for the benefit of another person without their knowledge - only applies when others who are present are not placed at a disadvantage or loss due to one's efforts for the other person.[2]
Saving a seat for another person on a bus often results in disadvantage or loss to the other passengers. For example, it is certainly deemed to be a "loss" if other paying passengers are forced to stand, along with the safety risks in doing so, because one is saving a seat for a friend who has yet to board the bus. Therefore, saving a seat in such a situation would not be permitted. However, in the event that there are still numerous comfortable and otherwise desirable seats available for other passengers, one would be permitted to save a seat for a friend.
Of course, one also has the option of simply paying upfront for more than one seat on the bus upon boarding. In this way one will have legitimately acquired the rights to all such seats. One is then permitted to use the seats in any manner one chooses, including reserving them for others who have yet to board the bus. This is true regardless of the inconvenience or discomfort of other passengers. One can then be reimbursed for the bus fare by the person for whom one saved the seat.
[1] This chapter is based on Mishpetei Hatorah by Rabbi Tzvi Shpitz, 1:85. See also Dvar Chevron 1:24.
[2] Bava Metzia 12a, Rambam, Hilchot Zechia U'matana Chapter 4, CM 105:90.