I would like to extend the Sosnytskiy answer and to tell my expressions about these two sentences.

When someone approaches me and asks "Хочеш купити спідницю?", I will more often tend to think that he/she are interested in me buying the skirt from them. If they ask "Ти хочеш купити спідницю?" - I will more often tend to think that they are asking just in general, not necessarily having some interest in me buying the skirt.

I do not know whether these versions are accepted here, but yes, "Хочеш купити..." and "Ти хочеш купити..." mean the same and both are gramatically correct.