Make sure you have a paper trail of who owned the amp. If a full paper trail is not available, then the warranty is null in mesa's eyes. They want to make sure no "non mesa" dealers have had the amp.
What I do is call mesa before I lay out the coin and make sure I can get the warranty transferred, once I have confirmed that, I make the purchase. If I cannot get the warranty transferred, I will not pay full price for it as the warranty is worth quite a bit in my eyes.