I do not build computers for customers and wouldn't do it unless it was a very good customer, they needed something specialized, they understood they would be over paying and they knew I wouldn't warranty it.
However, want to know what almost 40% of my business was last year? Helping customers buy new computers and setting them up. When I started my business I told people that this was a needed service and I could make alot of money doing it. Surprisingly I was right about something!lol
Here's how it works..... Customer wants to buy a new computer and doesn't know what to get. 70% of my customers still think a good laptop cost $1,200 and a desktop cost $900-$1,000. I guess they've all been shopping at Best Buy.

I then give them a ballpark of what I can find them a computer for, which is usually $450-$500 for a desktop and $550ish for a laptop. I offer to do a quick "consultation" about what they need in a computer, find out they will do with it, I find out if they need Office, Quickbooks etc. I find out what brand they like, don't like or if they even care. I explain the pros/cons of the different size laptops if that's what they are wanting. Then I go to Dell's Small Business site and price out the best deal I can find for them. If they want another brand I'll price that out too, but I don't go to brick and mortor stores bargain hunting or anything like that. I've only bought 2 non-Dell pcs for customers in almost 4 years.
I call them with the price and let them know what I've found. They say go for it, I order it. It bills to me, it ships to me and I unbox it, perform the out of box setup, create user accts., install an AV of their choice, install all Windows security updates and put it back in the box. I call them and they come pay me the actual cost of the computer and $75 as a "consulting" fee. It might sound like alot of work but it's really not any work at all. 15 mins consulting, 15 mins ordering, 15-20 actual work mins. setting it up and I'm done.
Here are some other tidbits of info....
I can rarely beat Dell's Small Business site when ordering a custom, name brand pc online.
I don't have to charge sales tax on the machine because it is paid at the time of purchase and I am only being reimbursed for the purchase. I contacted my state comptroller about this for clarification.
I won't pay for the computer up front unless I know the customer already or know them to be trustworthy. ie: you aren't walking in off the street and getting that offer.
I charge more for data transfer, installing other software etc.
I charge to come out and set it all up onsite if they like.
I am now "the" go to guy for so many of my customers when they want a computer. Residential and business customers alike just call me and say "hey I need a new computer, just get me what I need, don't even call me with the price, I know you'll get me the best deal".
I tell my customers, "I'm a tightwad who likes quality for the best price possible. I work for you, so I don't make a nickel more if you spend $1,000, my goal is to save you every penny I can." I prove this to them and they keep coming back.
I love it, it builds up a strong relationship with your customers and it leads to so much more business. I market this as "pre-purchase advice" and "computer purchasing help".