A reader of Paul Boag’s site asks if it’s okay to use a pre-built WordPress theme for a client site. Two people share pros and cons. The much shorter answer is, yes. It’s okay. But, you should absolutely disclose it.
And you should also consider whether it will really save you time. If you want to use the theme exactly as it’s intended, I think it can. Otherwise, it’s tricky. I highly recommend using themes from people you trust and whose code you know.