Good advice from Woody.
This happens to be my primary practice area so I suppose I should chip in.
For the will to be "legal" it simply has to be executed in accordance with the decedent's state law. Generally, that means exexuted by the testator in the presence of two (some states require three) disinterested witnesses who attest the will in the testator's, and each other's, presence.
Of course just because the will is "legal" doesn't mean it will do what you want or hope it will.
And also keep in mind that a will only directs property titled in a decedent's sole name -- it has no effect on property in joint tenancy (with some exceptions), property with a valid beneficiary designation or property held in a trust. But if you think you don't need a will because everything is joint tenancy, consider what would happen if you and your joint tenant die in a common accident, or before you have a chance to do a will. In addition, a will is where you name who you want to be guardian of your minor children.
Also, an estate probated without a will is usually more costly because a surety bond is required, whereas it can be waived by the will.
As for the sportsbooks, I'd just go with what Woody said. I can't imagine the sportsbook reaction if an executor actually tried to present letters of office to claim a sportsbook account. Might be legal situs issues as well.