Well legally you can sell concert tickets or other event tickets, you're just not supposed to be able to profit from them by selling them above the face value (I think there's a tiny bit of wiggle room, like 3% or so to cover whatever you may have paid for S&H). It would seem silly to "scalp" theme park tickets since it's not like they're going to stop selling them. The only way I can think of that being worthwhile is if you bought a bunch of passes now and then sat on them until WDW raised prices to a point where you could sell your tix at a markup and still be cheaper than WDW, almost like what the big bulk sellers like Undercover Tourist and Maple Leaf does but with smaller volume and more time.
To the OP, as others have said, it probably depends on the sorts of tix that were given to your brother. If they are generic tix that anyone could use, they'll never know. If they already have his name or some defining element to them, they can't.