buy (something) back (from someone)
To repurchase something that one has sold. For the right price, I'm sure you'll be able to buy back your old house. Once the dealer realized how valuable that painting actually was, he wanted to buy it back from me.
buy something back (from someone)
to repurchase something that one has previously sold from the person who bought it. Can I buy it back from you? I have decided I need it. He bought back his book from George.