If you replace your chain often enough, your freewheel won't get worn to the point that it only works with the stretched chain you are replacing. Usually in real life, you'll find you need a new freewheel when you change your chain. Whoever is selling used drivetrain parts on ebay is a crook and you got took. If the used 7sp cogs work with a new chain, it has very little wear on it and maybe you weren't robbed. Your used chain and used 7sp cogs weren't used together and chances of working together are virtually nil.