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.
I agree that it is ethical to use pre-built themes in client work, provided you disclose the decision and explain the relative merits (cost & speed being two common reasons).
Beyond doing the right thing by disclosing it to your clients, you can also use the disclosure as an opportunity to explain the merits of each (pre-built vs. custom), quantify the costs of a project, and also differentiate your own custom work by describing how it will be different/better for the project.