I never go into a bike shop to buy something of any consequence unless I've already done some of my own homework about it. That usually consists of a fair amount of research online. While some shops really know their stuff and even when they do, they often don't have the time to pour over my options with me. So, while I understand your frustration, you'll do better if you have a better sense of what you're looking for if you do some independent research about it. Then, find a mechanic you like and stick with them, being mindful of the fact that you'll get better service if you schedule it in advance.