Painting Prefinished baseboard
I don't think that's a very good idea. First of all, the wood is already sealed, so the paint isn't going to adhere very well. Second, you're going to waste a lot of money by purchasing nice grainy oak baseboard, since you're paying for the color and grain that are going to be covered up by paint. You can save a dollar per linear foot or more by going with some finger-jointed pine or MDF pre-primed white baseboard that you then finish off with a coat of paint.