I emailed them, and this was their response!
"Hello,Here is physical properties for our product:Use Consistency 26-28 cc/100gSet Time 18-25 minutesMaximum Expansion ( 0.25%-100 Mesh 99.5-100%Dry Compressive Strength 10,000 psiYes, it is identical to USG's Hydrostone.Thank you,YuriiPlaster Guys215-836-2413"
"Identical to" seems to indicate that it is not USG's product. Since USG owns the trademark to "Hydrostone," what they're doing by advertising it as "Hydrostone" is kind of shady, even if there's nothing wrong with the product, itself.
If USG wanted, they could pretty much put these folks out of business by taking down their infringing eBay listings, making it more difficult for people like you and me to find their product. Worse, using a registered trademark to sell a non-genuine product opens them up to severe legal action.
Those were exactly my first thoughts. So, you'd agree that the product seems to be identical to that of USG's and fine to order?
I'm surprised these people aren't more careful.