My top concern is authenticity. I've become friends with a consignment store owner here in Toronto, and she has had so many fake Hermes bags come in for evaluation. Many of them have vintage stamps. It's often easier to pass a "vintage" fake off as real since it can be harder to authenticate them. I myself would be vary wary of buying a very vintage bag, and would immediately have it authenticated or hotfoot it into Hermes for spa.
Sorry, but that's nonsense. Don't know how your friend is authenticating but stamps are only a tiny fraction of what's looked for.