Looks like its from the Acanthurus family. I'm basing this on the shape of the face, as well as the dorsal and ventral fins.
It also seems to have the same lines as the bristletooth tangs, or the Ctenochaetus family.
Based on fishbase.org it looks to me like it may be one of the following. Click on the photo in the description for additional images. My best guess is grammoptilus at this point.
Acanthurus grammoptilus (
http://fishbase.org/Summary/speciesS...e=grammoptilus)
Acanthurus randalli (
http://fishbase.org/Summary/speciesS...sname=randalli)
What was the fish sold to you as?