I know Xotic PC allows you to buy barebone systems from their website. For the most part barebone systems come as a kit including the laptop itself, the screen, motherboard, disk drive and video(if it's an option). Thus leaving it up to you to buy RAM, a HDD, and the CPU. Most people don't go through the hassle of building it themselves because laptops are nowhere near as easy to work on as desktops. If you are willing to put in the time and effort, power to you. I'm not too sure of any other barebone dealers, most build an actual system for you. Barebone laptops that I have seen, if you want to do more research, are ASUS, Compal and MSI.
FYI: most system builders like Dell, HP and cyberpower, do exactly what you are doing. They buy barebone laptop systems and add parts to customer order.