When I did this years back, I never attempted to promise to finish a system in less than a week, so I had time to order parts online. I always explained from the start that it would be a few days because I was getting better, cheaper parts. If they really wanted to spend more money to get it sooner, I just pointed them to a B&M store to buy a pre-packaged computer.
I usually researched common part prices and presented potential clients with some starting point systems with price/labor breakdowns. Then if they wanted something specific beyond that, I could give them a rough estimate, and provide them with a firm estimate in a few hours. I always provided final part price breakdowns and all the documentation that came with each part. I didn't mark up parts, and this way I didn't need to charge sales tax. I made it clear that I did not provide any kind of warranty or free support beyond that provided for the individual parts, but I did set up the computer in their home and show them how to use it for free. Generally my clients were small-business types who wanted a simple system without bloatware, or people with video-editing hobbies (this was back when you needed a really special gonzo system to edit video).