Is this a good deal?

Not great, but OK. At that budget, I think you could come close to new outside of this deal.

I don't agree that a 51cm would fit anyone 5'7" to 5'11" either (likely for a shorter rider), but fit is always rider dependant; no broad generalization like that should dictate a buy / not buy decision.

I would look elsewhere. You can get a brand new P2C DA for $3K or Utlegra for $2.5K and get a nice set of wheels. Size 51 for Cervelo is more likely to fit someone closer to 5 and a half feet than six feet.