Ownership
The two things that protect you are the serial number and the published specification. Where a manufacturer has stopped publishing specs — as Hyperice has for the Hypervolt 2 — a seller's claims cannot be checked against anything.
Undocumented
01 MODEL
Warranty transfers
CHECK
Serial
REQUIRED
Spec check
USE IT
The most useful thing you can do with a listing is compare it to the manufacturer's own specification. If a seller says a Hypervolt 3 does 3200 RPM, the Hyperice page says 1500–2500 and the listing is wrong. If a seller quotes an amplitude figure for any Theragun or Hypervolt, it is invented, because no model from either brand publishes one.
The Hypervolt 2 is the exception, and it matters here. Hyperice's page for it now carries a struck-through price and marketing copy with no speeds, no rate, no runtime and no weight. Nothing a seller tells you about a used Hypervolt 2 can be checked against a manufacturer figure, because there is no longer one to check against.
Worth knowing
Generation naming is a genuine trap on the Theragun line. Prime and Prime Gen 6 are not interchangeable in a listing, and a seller may not know the difference themselves.