You wouldn't use the phrase "buy to" in normal speech unless you were using "to" to mean "in order to" and the object (what you were buying) was already known or articulated. Even then it's slightly awkward and not that common. An example might be "See this house? If I was you I would buy to make a profit right now" or "There's a gadget I saw, it's something you could buy to give to your son" etc. You would more often say "buy it (or the thing) to" and that's a lot more natural. "Buy for" is similar although more common, as long as you mean buy on behalf of / to give to as opposed to buy because. "There's a ticket I'd like to buy for a friend". 'Buy it for' is still much safer and straightforward.