If you are serious about fixing phones, then you need to start with a supplier/vendor that will stand behind you. The hardest part of building a new business is finding reliable suppliers. It really hurts you're profitability when you end up having bad or poor quality replacement parts because now you are stuck doing "free" warranty work and you may have to eat the cost of the defective parts.
A good supplier will stand behind you whereas a poor one will always find an excuse to not honour their own warranty or impose difficult hurdles to getting parts replaced or refunded. That is why I don't recommend you use sources like Aliexpress. While they do offer reviews and seller recourse, you are stuck with shipping defective parts back to China. I have had a few situations where I had to do this and it was very expensive shipping parts back, the timelines where extremely long and the seller was even challenging my request for reimbursement. In the end, it took several months to get my refund. That can be a killer for a small business.
eBay is better because sellers care a lot about their reputation so they will almost always settle quickly. But here is a little known fact about iPhone replacement parts. Everything that is manufactured in China is eventually sold. The best stuff goes to Apple, the good stuff is picked up by large vendors who sell to large customers and as the quality goes down, so too does the quality of the vendor. That is why sites like eBay, Amazon and Aliexpress are rife with poor quality parts. You can find some good stuff but you will have to experiment to see who can offer you good parts. Again, for a small business, this can be a killer.
So I would focus on established vendors, who offer lifetime warranty and will work with you no questions asked when you have issues. Depending on your location, look at companies such as iFixit, MobileSentrix, Mobile Defenders etc. They may be a little more expensive than Aliexpress but long-term you will save money. If you ever get to be a large volume shop, then you can establish you're own relationship and have the leverage necessary to get premium service out of China.