The best way to show professionalism and respect is not to waste the listener's time. Let them know you're good by providing an example of your service for them. This takes your experience and references out of the equation, and eliminates the risk of them giving you a shot. It also differentiates you from 99% of your competitors who are too lazy to do this.

For example, if I were a copyrighter, I might write:

"Bob, I'm a professional copyrighter specializing in helping online artists get more sales for their art. I've rewritten listing #17 for you in accordance with proven marketing principles to help you get a greater response. Try it out, and if it works for you, the only compensation I ask for is 2 referrals to other good artists you think would appreciate my service. And if you want additional listings rewritten, I'd be happy to redo them for $50 each."

Once you get your first few clients this way, you'll have to do less of this and can rely more heavily on your references.

Good luck!