A bookseller is a person or a company whose work is selling books. That can be the owner of a small shop, someone behind the till, or a big chain. The word is ordinary and a little formal — in speech people often just say bookshop or bookstore for the place, and “the people at the bookshop” for the humans. Though actually, bookseller can name the business itself, not only one person standing there. It is not a librarian (who lends books in a library) and not a publisher (who makes the book and sends it out). It has been in English a long time; it is not slang and not academic.