Q: Do I need an agent to sell my book proposal?

A: It depends.
This is almost like me asking you if I need a realtor to sell my house. Okay, in light of the current housing market, that might not be a very nice parallel. But then again, a quick look at the publishing industry might make that comparison even more apt. But back to the question and topic.
In the "old days" of publishing, let's say prior to 1990, there was a common publishing phrase that referred to an unsolicited manuscript that was sent to a publisher as something that "came in over the transom." (A transom is literally a hinged window over a door. Think of the book return slot at a library.) In other words, a writer sent in his or her manuscript to a mail drop, which then ended up in one of several 4-foot high stacks in a junior editor's office, and which after six or seven months of collecting dust was either rejected with a form letter - or voila, it got discovered and published. One way many publishing companies handled submissions that came over the transom was to hire college interns to sift through hundreds or thousands of manuscripts over summer break and separate the winners from the losers. More