It's happened to me on another site, and telling it to other writers confirmed this wasn't some one-time deal.
I'd be also wary of artists who do way too much tracing - Greg Land levels, or constantly imitating popular poses for their work. Some might be fine for reference or homage, but nothing but these two can mean how the artist might be planning on getting your commission and running off with it with nothing to show for it other than someone else's art slightly touched up.