Greetings Michael,
There are multiple ways one can market single trees: local newspaper, sawmill directory, provincial timber buyer list, word of mouth, or contact a local professional forester to assist you.
Timber pricing and veneer estimates are often subjective, especially if you only have one tree - even more so if the tree is a "yard" tree. Walnut veneer prices may be anywhere from $3.00-15.00/board foot. Again, very subjective and not easy to quantify without knowing the true log quality attributes.
My suggestion to you is to contact your local Ontario Ministry of Natural Resources office for technical forestry assistance in Canada http://ontariosforests.mnr.gov.on.ca/aboutus.cfm
Best of luck!