Is a handshake promise from the seller worth anything if it's not in the signed agreement?

Generally, not much, if the purchase agreement contains an "entire agreement" clause, which most do — this kind of clause states that the written contract represents the complete deal and supersedes earlier discussions, precisely to stop one side from later relying on an earlier oral promise that never made it into the final document. A verbal assurance the seller gave during negotiations, however sincerely meant, usually can't be enforced as a separate promise once that clause is in place.

There can be narrow exceptions, particularly where the oral statement amounted to a fraudulent misrepresentation that actually induced you to sign the agreement in the first place — courts are sometimes willing to look past an entire agreement clause in that specific situation, though it's a genuinely harder case to make than simply pointing to what was said. The practical lesson cuts both ways: don't count on that exception rescuing something important, and make sure any promise that actually matters to you gets written into the signed agreement itself rather than left as a verbal understanding.