Hi
As Flopps says theres loads of people at it, just have a look on ebay at all the companies building computers.
The only way you are gonna make any cash at all is by setting up you're own company and not paying the VAT on the parts you buy.
I asked friends and family to get in touch if they knew anyone who was looking for a new computer and it has worked quite well over the last 5 months, its often cheaper to build them yourself.
All i did was get in touch with some component distribuitors and ask what deals you can get for large or bulk buys, works quite well as at the moment i get 10% off all purchases and free delivery.
After building about 50 computers so far it is starting to dry up, but i am stopping anyway as most people don't have a clue about computers and as you built it they expect you to sort it out.
All the PCs i built were from new but if you are up at 1am then i have gotten some absolute bargains on ebay as most are in bed, i only use these bits to build budget PCs for my mates kids who are just gonna wreck it anyway.
For £80 i got;
Intel Dualcore 3.0Ghz (1st Dual Core)
Motherboard with integrated graphics
2GB RAM
DVD
80GB Hard drive
Case and Power Supply.
As i'm leaving my job for a new one, i have been stuck on nights for nearly 2 months so am free to bid when no-one else is.
Good luck and hope it helps.
Oh try Freecycle and the local tip.