I think most people, ie, non-gamers,would see a game and say, "Five bucks off, and still guaranteed? Sure!" because they may not have the time or interest in stretching the bucks as we do.
Used games are definitely more profitable, for the same reason (someone obviously wants seven dollars credit more than his game that he's already beaten).
Nowadays instruction manuals aren't really even that necessary--most games have some sort of tutorial level, or there's always gamefaqs. But again, most of us are probably completists, or at least are thinking of future sale/trade when buying a game, so it's definitely an issue for us; may not be for Joe Consumer.
But yeah, the prices are not attractive, if it's less than 10% difference I'll usually buy new to get guaranteed complete, esp. when you can order from some places online and not pay tax, and I do like games to be complete.
And I can kinda see why the manager can't drop the price of an incomplete used game. For most corporate endeavours, pricing comes from on-high, at least at the district level if not state, or at least above the store level. Removing that level of decision making helps the business run consistently (if not necessarily "efficiently"), and helps to eliminate the possibility of fraud.