@Emil - I've seen a lot of great ideas mentioned in the comments like a review seal, public reviews, and quotes. While all those ideas are great, it's sometimes best not to take too much on at once. Take some time to incrementally build these things. Let the business and feedback from your clients help steer the direction. Ultimately, the business should probably be about giving your clients peace of mind when shipping their code. They want to ship good code, and you're the guy that's going to get them there.

Anyways, I have a couple of other ideas:

1) Have a reduced price for version upgrades. Repeat clients are probably going to be a big deal in sustaining the business. It'd be nice if everyone in the theme community would use Semantic Versioning. It'd make pricing upgrade reviews easier.

2) Utilize GitHub in the review process if possible. It makes things so much easier.