I'm curious - who regulates the computer building industry and makes a license a requirement?
As most have said you can build computers fairly easily and if you're just doing it as a hobby/for pocket change then that's fine. It's not something I would really recommend as anything more than that - it's also worth remembering that in building the computer you will almost certainly become technical support for these people from then on which can last well beyond the point where you've made any profit on the deal at all.
If I was going to build computers again I'd probably just do so for friends, charge them a reasonable amount (or as I used to do leave it up to them) and then once I'd exhausted that market stop - the returns for trying to expand simply weren't worth the additional effort.
You need to answer the questions from my above post first. And if you still think it's possible work out how much someone could buy it off the shelf for, say from Dell for example, and then work out how much you could do it for. Allowing for you to test it, the time to build it and the extra postage costs.
I reckon your postage costs will be £20 easily, plus you need to find a courier that will send it for you insured.
I can think of dozens of problems. You haven't given this any thought really.
You need to do a lot of thinking.
Yes they do? They even have a brand dedicated to the concept of "high end" tech, not that it's particularly good.
Its not really going to go far. If your mate already wants a computer anyway, say you'll make it for 20 quid if he buys the parts (although then you're taking on the responsibility if you break it of paying it/ broken friendship etc). As above, not worth it. Not a great entrepreneurial idea ;D.