Pokémon that originate from a Gen 3, 4, or 5 game won’t have an origin marking. The only games within that period where Groudon is catchable in are Ruby, Emerald, and SoulSilver. Check where it says it was met. If it says “Traveled through both time and space to reach you from Hoenn / Johto” then it’s from either Ruby/Emerald or SoulSilver, respectively, and could be legit (if it says anything other than one of those two, it’s definitely hacked). Although I don’t trust a seller on EBay, so even if it could be legit, there’s still a pretty good chance that is was hacked, especially considering that they told you it’s from USUM.