Can anyone come and sell books there or is it just for engineering students?

It's run by the EngSociety and advertised to engineering students. If you had a book that would be relevant to Eng students, no one would turn you down to try and sell it, but the crowd will be looking for books for common eng courses. Virtually no other students show up.

If you're in Life Sci, you could always bug the Science student society Executive to plan one of their own.