Anyone know??? Thanks Gary

All of them!

If its a "PRIVATE" sale how would anyone else know.

I believe it depends on you income from the guns.  Check the ATF site.  If you make over a certain percentage of your income that year from "private" firearms sales, they will get you for dealing without a license.  I don't think its jail time, just big fines(I suppose they view it as you trying to get around taxes and the FFL fee).

Why would you sell your guns?

It depends on how many you have to sell.

It depends on which State you live in. For example if you lived in Massachusetts you have to fill out a blue card to transfer a gun to a private party to register it in his name. I believe there is a limit also of 4 firearms per year. Someone from Mass can chime in with any changes but that's how it was when I lived there.
Here in Arizona there is no paperwork required on a private sale....so you can sell trade whatever you want and as many as you want.

John