Use an example of their attributes. Ie. Quality, knowledge, promptness, how he treats you as a customer and others. You can use an example of some repair that you had done by that person. Is his shop organized? Does he go to extra effort to make sure the job is done correctly. Just as if he is working for you as an employee and you are evaluating him/her. Keep in mind that you are recommending him to someone else, only in a letter form. If you don't feel you can recommend the person to others then a letter of recommendation is not appropriate.