If you are referring to the regular 1 year warranty, that expires after one year whether using it or not. If the seller bought the extended Applecare Protection Plan, the seller had to register it within 364 days of date of purchase - if he did that, make sure you get a sales contract/receipt with his name, address, etc., the Mac's serial number and the selling price and keep that for proof that the machine is covered (also check online or call Apple to make sure).
And no, it may not cover anything from non-usage necessarily - it would depend on the particular problem.