I'd give it a shot if you upload a clear image of it. Unless it has clear male characteristics, identifying the gender from the exterior is next to impossible. The easiest way is to take a look at the inside of the abdomen of a molting. Telling the species may or may not be easy as well.

I'd also advice against buying a tarantula from a shop that doesn't even know what species it is. They wouldn't be able to tell you anything about how to take care of it.