^What they said. It should be alert, but it shouldn't seem overly stressed. Any mucous around the eyes, nose, mouth, or vent is a red flag. Also check for mites. While they aren't as common in geckos as they are in snakes, they can get these little red ones in the folds of their skin. Speaking of which, there shouldn't be any wrinkles in his belly. You want a nice and plump guy, but not a fat one.

Ask the vendor whatever comes to mind. Interrogate him/her. If there is any hesitation in an answer, think twice about who you're buying from.

Good luck!