It really depends on the condition of the head, what options it has. I'm always on the lookout for another Mark III Coliseum. I currently have a Blue Stripe Coliseum and would like to get a backup! If you are serious about selling, please send me a PM!
You wouldn't believe what I picked my Blue Stripe up for, but lately I've seen them selling anywhere from $900 to $1,200. The highest I've see. Then sell for was around $1,500, but that's the top of the Mark I've seen, and I've been on the lookout for the past 7 years or so.
But seriously, if you're interested, I'm most likely a buyer! Cheers!