Wow if you have to even ask that question you aren't a capitalist.

Not your fault some sap was willing to sell you something for $2 that you can sell for $70.

Whatever the market will bear. You shouldn't put a limit on your profit, just make the asking price what you think you can get for it.

If I am paying under $45, then I could not care less about "profit" for the used game retailer. In my case and many others it would be Amazon.com and Gamestop. You want me to pay full price? Then put out a game on par with Red Dead Redemption, Fallout 3 or Batman: Arkham Asylum/City. Otherwise, I buy your product used, which means no profit to you.

Unless you are forcing people to buy something or you are commiting fraud I don't see how morality comes into play here. If you can sell them at a higher price then more power to you.

Whoever buys your game obviouly values a rare game more than their $70 or whatever they are paying. You gain the money you wanted and they gain a rare game they wanted ... who is the loser in this situation?

Go for it and sell for as high as you believe people will pay.