1) an expert in usability and have few papers written on it
2) have many years of actual usability studies and design
3) have designed interfaces yourself and have examples of UIs you've designed.
Just getting testimonials for 'reduced pricing' won't cut it. What you need is a resume/cv. All these big usability experts sell themselves first and their work second. You need to sell people on why they should trust you.
This startup is basically just selling of your services. Here's some ideas. Start by writing whitepapers & studies of other sites. Do them for free. Just pick any known site and say what you'd do differently. Do this for a lot of them. Make a blog and do a study every day. Make a wiki with usability practices. Start recording videos of random users using some site and criticize them.
In short, work on your credibility since that's what you're selling.
The fact is, I am not a big usability expert. I have given a couple talks on the subject and written a few blog articles on the subject (one of which A List Apart was interested in publishing). I've also done a number of usability tests for companies that I have worked for. Therefore, my thinking is that a more feasible approach in the short term is to show that I actually produce good results. Establishing that I'm an expert is something that will take time. Thankfully, I enjoy usability and will enjoy establishing expert status. In the meantime, if you have any suggestions for otherwise convincing people that they should go with me, I would appreciate it.
It's interesting to me that nobody really cares about the "usefulness guarantee".
Also the block on "how usability helps" basically says nothing. It might as well just say, "Usability is good!" You're going to have to fork over a lot stronger value proposition than that before getting someone to pay for the service. Don't just get reference customers; get reference customer numbers. "With Really Cool Customer Foo, signups went up by 128% within two weeks."
The selling yourself bit, I'll be a little harsher (in hopes of helping), it basically says, "I've done this for some of my buddies sites and they think it's neat." Again, not a strong value proposition. Just cut that part out if you don't have a better case to make; that portion actually makes you sound less qualified that I would have assumed by default.
But hey, I'm no usability expert either.
I didn't notice this at first. It's very useful advice. Some of it I've already started (writing whitepapers & studies). My friend and I have been talking about putting together a wiki with usability practices.
In the mean time, I have had a few people request the service because their web developer (whom I showed the site to) recommended it to them. Hopefully word of mouth will continue to bring in business and increase my cash flow. Over time it will be great to build my credibility in the ways you mention. In the short term, it's also pretty important to me to start gaining paying business.
Having a blog also makes you an authority, which gets you to expert status where people are comfortable paying you for your services. I know blogs are a lot of work, but once you get going and have been doing it for a while it really pays off.