I think you need to find out a lot more about cannons before you buy. Do you research before buying. Buying cannons is not something that you should not take lightly. A decision based on price alone is a wrong decision.
You need to ask how the breech plug is installed. Ask him if the breech plug and liner complies with N-SSA standards. I believe the N-SSA regulation require pictures of the breechplug. Ask if the liner is seamless tube.
What is the diameter of the bore and the diameter of the barrel around the chamber?
Remember when you fire a cannon you are setting off a controlled explosion, you want to do that safely.