Typically, you get what you pay for in terms of Iphone replacement LCD's. You could have purchased an inexpensive replacement part or have a defective LCD. Open the phone and look for the Apple Logo on the LCD Flex cable is one way to tell. If you spend $30 - $50 on an iphone 7 LCD, you bought a third party knock off and quality will not be the same as a $100 + Apple original LCD.
You can also ask the seller where he gets his parts and the quality. There is a bit of confusion on Original Apple LCD's, OEM, Class A and inferior knock offs. Different people have their ways of describing it and can be misleading.