If the "hard soil" underneath is clay, tilling it is not really going to have any effect.
Also, putting down enough top soil to get the sod to take and laying new sod is going to be an expensive endeavour...make sure you're going to get your money's worth out of it before doing it. In this current market, it likely isn't going to be a deal breaker on selling your house. If it is really a big deal to the prospective buyer, they'll likely get you to knock the purchase price down a bit.
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