For me, selling is easy when I really believe in what I am selling. If I know that I can do the job and do it really well, I feel comfortable asking for my rate.
When I first started training, I was not as experienced and quoted much lower rates. I would sometimes take a dog for a week or two of training in my home and charge about 2x my hourly rate per week (something I would never do now). But that let me get my hands on dogs that I was having problems with and also just get more experience.
I also had a harder time knowing how long it would take for me to train a given dog. At that time I quoted a price and estimated the lessons, but agreed to keep working until the training was done. This way I could be sure I wasn't over-charging or over-promising.
One thing I do not do, is sell something I don't believe in. If I am not sure I can fix a problem, I tell people that up front. Especially with aggression, the outcomes are not always predictable. I do not make guarantees when I agree to train a dog with serious behavioral problems. And I make this clear to the owners. I think when you do this right, people actually trust you more, bc they know you are telling the truth.
I also notice when my prices seem too high for people. Sometimes I will give the people a lower price or let them pay for a package over time to make it easier for them. I might also agree to a lower rate if they meet me near my house (I usually do lessons in their homes). Other times I will recommend that they try a group class that is more reasonably priced. People appreciate it when you would rather give the business away rather than continuing to sell them on something they cannot afford.