Decline is housing sales would be due to many reasons, however one can expect that to be an opportunity to buy a house with a good price, if there is a decline in housing sales and there is surplus in supply, which brings us to supply and demand analysis.
Supply and demand is a very essential factor in sales and purchases and housing sector is not an exception. So, if there is more supply than demand, one may expect the prices to be lower than usual and that would be a good time to buy if there is a need to buy. However, it is not possible to state that a decline in housing sales would solely be an indicator of an opportunity to buy a house; simply because there are many other factors. Comparison of cost and profit is one of them.
Cost is crucial, because it signifies how low the housing prices can go. Contractors may be willing to sell a house for prices lower than what they usually sell, however they would almost never be willing to sell lower than what the houses cost them unless there is an unusual need. Thus, knowledge of the cost of the houses as well as knowledge of a minimum profit to be added on top of the cost of a house would give the buyer an idea about what prices could be considered an opportunity.
However, decline in housing sales may coincide with the decline in the sales of other commodities as they commonly do, thus the cost of the houses would be less than usual as well, even if the said lower costs are not reflected in the housing prices by the contractors and that would be a motivation for the contractors to go for lower prices in bargaining even if the prices actually remain the same as before.