Ballet slippers/shoes should be fitted from the back of the slipper/shoe, ask the child to push their toes to the front, and then point their toe forward and you should be able to pinch the heel seam, if you can pinch the whole of the seam then its a good fit, if you are unable to pinch anything they are too small. Yes, the size should be 1 - 1/2 sizes up from normal shoe size. One foot is nearly always bigger than the other so obviously go by the biggest foot. Also remember you can always buy wide fitting ballet shoes should you need to.
I hope this helps anyone who's thinking of buying ballet slippers/shoes the e-bay way.