Provincial property assessments should never be solely relied upon as a relevant indicator of true market value for the purposes of purchase, sale or financing. It can be a starting point, but you should always acquire the help of a professional as well. It is important to note that valuing your property is not something you should do on your own. In a buying/selling transaction, the best course of action is to order an appraisal. Professional appraisals are a much more accurate reflection of current market value.
In general, market value is determined by what a buyer is willing to pay for a home, and what the seller is willing to accept. A quick survey of recent sales and their relation to assessed property values demonstrates no clear relationship between sale price and assessed value – some properties will sell below their provincial assessment, and others far above.
In addition, you will want an experienced, local real estate agent to also help you determine the selling price of your property. This agent will have a much better idea of what is happening within real estate prices in your area than a government document would. Real estate agents are generally relied upon for an estimate of a listing price. Most buyers will only care what they believe the property is worth, which is why market value is usually determined by lining up what a buyer is willing to pay for the property, with what is acceptable to the seller.
Overall, you should gather professional opinions from real estate agent(s) as well as an appraiser. These are the people dealing directly with the market first-hand.