If he makes you a reasonable offer, you better get your hands on it. Nobody I know of has sold one and not regretted it. They have a bit of a cult following and can be highly sought after.
Double Tap is the only company I know of that loads the STW ammo up to its potential, though you can find good ammo that's been loaded down to 7 mag velocities. This thing really comes alive with hand loads, but the Double Tap with 160 Acubonds are fantastic as long as the rifle likes it. Mine does, and I suspect most Senderos will, too. My STW is a mutt, but it's set up similar.
At any rate, you would do well to relieve him of it.