"Sometimes he buys milk here."
Translation:On někdy kupuje mléko tady.
It is borderline and not that natural. It strongly stresses the verb. It is either an answer like "Sometimes he DOES buy milk here." or you can be stressing that sometimes he BUYS it here (and at other times he gets it for free instead or something similar). I will add it, but you should always be aware that Czech word order is free, but the shuffling usually changes the meaning.