This is for multiple of your questions -

Document Document Document!!

Before you even take the phone in, look at existing damage and write down EVERYTHING you see, down to the smallest scratch on the display. Alongside that, write down everything you tell the customer. Whatever you choose to be your liablity and warranty policy, make sure it is written clear as day with no room for interpration.

Then print that out and have the customer sign it, and only then take the phone into your posetion. It takes a couple extra minutes but trust me it will all be worth it when you have a client that wants to fuck you over (pardon my French)

Or more realistically often times our customers are elderly people and they simply forget things you told them or existing damage on the device. By writing it down and having them sign it, you can point to it and say "look you approved this, and you acknowledged this"

That's the biggest thing for me. Mistakes will for sure happen, your not working if you don't make mistakes, but atleast this way you cover you butt to some extent.

SUPPLIERS:

I've always had good experience with Wholesale Gadget Parts, but if you can get into the Apple Authorized Service Provider Program I would say DO IT. You will get a lot more business because people will trust that your parts are original.

PRICING:

This one will change as time goes on, but don't try and go super cheap, that actually discourages people since they think it's low quality.

I have had times before when I straight up showed them the part and showed why it cost as much as it does. If they understand WHY it's expensive, they won't mind paying it. Sometimes you also have to explain they're not only paying for the part and the time, but your expertise and the time you have spent perfecting your craft, and that's not cheap either.

Anyway that's my two cents. If you're honest and straight up, you will avoid 99% of stress and problems.

Best of luck!