I think you need to buy in good trusted places. Many sellers check it for quality and performance before sending the goods. In addition, if the spare part turns out to be of poor quality, you can return it to the seller. But I have been ordering everything from the same place for a long time. Never had a problem. Here you can see spare parts mobile, here chip price, huge assortment more 20,000 products. I like it))

A lot of Wholesale Suppliers sell the iPhone repair parts, If we buy the iPhone parts then How we can check the quality of iPhone parts?