Sell a computer you built? 
I just run 'em into the ground.
As for transferring. It depends on the license you get. Thumbing through the Microsoft EULA that came with one of our PCs, the license goes with the computer. The only time you can transfer the license is when you sell the computer.
For example, we own a large chunk of licenses for Windows 2000, that we can use on any computer. The desktops themselves come with an OS and license. We don't actually use that license, since often the machine came with something other than 2000, we use the pool that we bought. Therefore I have a bunch of Windows 98/ME/NT and old 95 licenses sitting around here. However, legally I can't give you one or sell you one because it legally is only good on that machine. If you build a machine, buy the OS and then sell them both, keeping no copying of the OS for yourslef and give them the EULA you should be okay with Microsoft.
I just run 'em into the ground.As for transferring. It depends on the license you get. Thumbing through the Microsoft EULA that came with one of our PCs, the license goes with the computer. The only time you can transfer the license is when you sell the computer.
For example, we own a large chunk of licenses for Windows 2000, that we can use on any computer. The desktops themselves come with an OS and license. We don't actually use that license, since often the machine came with something other than 2000, we use the pool that we bought. Therefore I have a bunch of Windows 98/ME/NT and old 95 licenses sitting around here. However, legally I can't give you one or sell you one because it legally is only good on that machine. If you build a machine, buy the OS and then sell them both, keeping no copying of the OS for yourslef and give them the EULA you should be okay with Microsoft.