Information on recent sales is not in any central location that I have found, and it's important that the sales examples be a close match to your property in size, number of bedrooms, etc.
There is ample information on the cities website that directs you to the NYS website that has further information.
There are real estate websites that go back three years with previous sales data that allows one to filter for the square footage, number of bedrooms, # of bathrooms, lot size, location...
The city/county clerk and assessors office has all sales data
A homeowner could call a local real estate agent to collect comparable sales data. A small fee may apply.
Call an appraiser to get the value of the home. A few hundred dollars for most homes. The appraiser will get the comparables for the homeowner.
The assessor can only base the value of the home by the exterior unless improvements have been made to the interior by a permit.
If the exterior is in better condition compared to the interior than most likely the assessment may be too high.
There may be a neighboring property that brings down the value of the home that the assessor may not have taken into account.
If the assessment has increased by a significant amount it is in the best interest of the homeowner to contest the assessment