What is salvage/rebuild in the title? Does this mean it was already a salvage before this wreck? Essentially salvaged twice?

Salvage/rebuild means it is currently having a salvage title. You would have to rebuild/reconstruct (Reconstruction procedures in USA state-by-state) in your state in order to receive a reconstructed/rebuilt title. Learn more about What is a Salvage Title?
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Does “Buy Now Price” means the vehicle is all mine if i can pay for it?

Yes, however, with buy now’s once you click BUY NOW, we will post an invoice for you. Once we receive the payment, we will attempt to buy that vehicle out for you (given it’s still available at auction). Buy now items are treated as a first-come, first-serve basis. Related: How it works “Buy Now Vehicle” option? Read more