Generally i don't like building computers for customers. It's a PITA if the computer runs into some hardware problems within a warranty period you define to the customer. You then have to deal with RMA on your own dime. I don't advertise computers, i just fix them. But if i run in to a very old computer that is not worth fixing, i recommend replacing it.
My main selling points for custom built computers are:
1. They're built with quality non-proprietary components that usually outlast those of OEM's. OEM's tend to nickle, and dime on things such as solid capacitors, higher quality HDD's, etc. If they can save $5 on a computer, they'll save millions on the whole line. With custom systems, you don't mind adding a couple more bucks for something better.
2. You have better support.
3. You can build a computer that fully suits your needs out of thousands of different interchangable parts.
On the other hand, OEM computers such as Dell, and Acer, are cheaper (at least for budget builds), and have warrenty, so it's less of a headache.
As for profits... i sell the computers at cost, and tack on $100 - $150 for setup and data transfer.