There's a difference between fixing bikes for your friends and running a small business out of your garage. It's the reason for a business liability insurance policy.

What if you don't properly tighten a brake cable fixing bolt and the first time the bike owner squeezes hard on the brakes, they pop loose and send him hurtling into a busy intersection. If he gets injured or worse, you get sued. Buying bulk bicycle parts will be the least of your concerns at that point.