Can I play Dominion: Hinterlands by itself?
Unfortunately, the clerk was mistaken. From Rio Grande Games' website:


  Dominion: Hinterlands is an expansion, and cannot be played by itself; to play with it, you need Dominion, or a standalone expansion to Dominion (e.g., Dominion: Intrigue). Those provide the Basic cards you need to play (Treasure, Victory, and Curse cards), as well as the full rules for setup and gameplay.


So your options are:


Buy the original Dominion
Buy Dominion: Intrigue
Buy Dominion: Base Cards (for the base cards) and download the game rules from their website


NB: Rio Grande Games' website says that the base cards are coming soon, but the set has actually been out since 2012

Ask for a refund




But in an alternate universe, it might have been a standalone…

The Secret History of Dominion, written by Dominion creator Donald X. Vaccarino, mentions that during the development of Hinterlands, they considered making it a standalone game:


  Hinterlands flirted with being a standalone. Intrigue was a standalone; Seaside had playmats and tokens; Prosperity had playmats and tokens.  Hinterlands and Dark Ages had no playmats or tokens. Would they just be slightly cheaper? Would what the price would have to be look attractive? That had been an issue before. I figured hey, one of the sets could be a standalone, let’s say Hinterlands, and the other could be a full 500 cards – no playmats, not a standalone, just gigantic. I had two solutions that let the sets be 500 cards.
  
  There were certain pros and cons to having Hinterlands be a standalone. In the end Jay decided against it. For a while I was trying to make the set simpler though, to make it function better as a standalone.