Your Realtor has this information readily on the multiple listing service for the area. Making the right offer at the right price is a good reason to hire a professional. Usually the seller pays the commission. However, don't be misled into thinking that the list price to sale price is necessarily a good measure of value. Some sellers want to list at the high end and some at the low end within a reasonable price range. Some Realtors tend to list higher and some do not. Therefore, just because there is an average percentage between list and sale prices does not mean that this is a reliable number to bank on. Your particular offer may need to be 15% lower or only 2% lower or even at asking price.
Study the sale price of recent sales instead and adjust the price of your intended purchase according to the differences between that property and the one that sold. If you would prefer, hire a property appraiser to give you the information you need to offer the correct price.
The seller, however, may or may not be reasonable about the value of their property either. This is a function of one buyer and one seller.