The end dates for the resort contracts are fixed, regardless of when they were purchased. For example, BWV contracts expire on Jan 31, 2042 whether you purchased there the first day it went on sale or you purchase it directly from Disney today. The same end date applies to BCV, HHI, VB, VWL and non-extended OKW contracts. SSR contracts expire Jan 31, 2054 and AKV contracts expire Jan 31, 2057. I believe the BLT contracts will expire Jan 31, 2060.
OKW contracts can have one of two end dates, either Jan 31, 2042 or Jan 31, 2057 if you purchase an extended contract. As Carol said, if you purchase on the resale market, you are purchasing an existing contract. So in the case of OKW, you would need to know whether you were purchasing a contract that had been extended or not. If you purchase OKW directly from Disney, they sell the extended contract.