I'd guess you're right, it's probably a Faber HP100.
Depending on the age of the tank, $200 is a lot for a used HP100. It may be ok if this is it's first hydro (meaning it's only 4 years old), but consider the possibility that it will fail hydro when you determine if it's worth it to you. If it had new Hydro, I'd go for it, but not with less than one year to go... if it fails, that's an expensive price tag for one years' use.
I'd say make the guy hydro the tank, then pay the full $200. Or at a minimum, make sure he'll give you a refund if it fails, and hydro it right away.
People say steel tanks never fail hydro, but that's what happened to me on a used tank I picked up for $160 at a swap meet. Score! But 9 months later, it failed hydro. Not such a score after all... the person overfilled the heck out of it.