An original Dunstall tank is basically unusable due to being made of poly resin which is not compatible with modern gas. The chip is through the gelcoat and into the fiberglass mat as well. I would say $100-200 max. Glass From the Past makes a replica Dunstall tank for the CB750 that is compatible with modern gas. You would be selling to someone wants an original for some reason not really for use.
If it was cheap enough, I might be interested in it to cut the bottom out and have a smaller steel tank made to fit inside. Let me know if you get it and what you want for it.
Scott