It would have to be a pretty tight-a buyer who wanted the postage costs back if you drop it off, getting it to them quicker than post. I wouldn't even offer a refund on postage.

I have recently bought something, and the seller is going to drop it off at a mate's place who lives close to them. They have charged $10 for this service (postage was gonna be $20), and i was happy with this deal. They have to take the time, and use petrol to get there.