Is it wrong to resell tickets to my girlfriend at a higher price?
What would make you want do sell it at a higher price?
I'm simply going to gain some extra cash because she doesn't know how much they actually cost.
Why do you feel that it would be more productive to exploit her for the money instead of being fair in price>
It's not that big of a deal.
You girlfriend might be hurt of she found out that she was overcharged.   Be honest with her about how much you paid and charge her that price, or even give her a discount since she's your girlfriend.