I don't have experience with StubHub and don't fully understand this sentence, but might the bolded portion mean that the seller must also be in the US (i.e. that the seller must have listed the tickets through the US site)? If it is, it's poorly drafted.
Potentially, a bit dissapointing if that is the case. Typically market wording is spelt out pretty good.
I will update on August 12th once I get a reply.