How do you tell what brand/make/model of bike I have? I have a bike I'm trying to sell because it's just been sitting in my basement. It looks as if someone primered it and the only thing I found was a serial number. Is there like a website I can look up more info on the bike using just that number?
The serial number on its own is basically useless. They're not like VINs on cars: there's no global database of them and they're not even guaranteed to be unique. Each manufacturer of frames uses whatever scheme they want, and the serial numbers are purely for their own stock control.

To identify the frame, you need to compare its appearance to frames whose manufacturer you know. However, unless there's something very distinctive about your particular frame, it's unlikely that anyone will manage to identify it. Most frames have nothing particularly special about them, and many are made in enormous factories in China that sell frames to whatever brand wants to buy.

If you've identified a small number of possible manufacturers, the format of the serial number might allow you to tell which of them is correct. But even then, it's something of a long shot.

See also, Why shouldn't I care what model/make/year my bicycle is?