If the wheel is centred when the screws aren't even, that means the frame isn't right and/or the wheel's dish is off, and/or the axle is bent.

If it's a freehub, it's not likely to be the axle, but check it by loosening the QR and turning the axle in the frame. If this doesn't make the wheel doesn't wobble around, you can flip the wheel to check the dish. If the rim sits in the same spot each way, whether that's centred or not, the wheel is properly dished.

If the wheel is properly dished and has a straight axle (or you can twirl a bent one for the average), only then can it serve to indicate the straightness of your frame.

I think I'd be hopping mad at that seller, BTW...