> Is selling something that isn't even built yet common?

When Microsoft "sold" their operating system to IBM to run on the future IBM PC's. They didn't own an operating system, they didn't have a working operating system. At that point they had wrote a few apps for Macs, and a BASIC compiler.

One of their co-founders knew a guy, who might sell them his, maybe, for a good price.

I'm not joking. Stuff that doesn't exist gets sold every day.