iPhone 7 LCD Screen and Digitizer

I can't speak for iFixit but I have ordered parts here in the past, and continue to do so. The main difference is written up over on this page. In a nutshell, you are paying for a vendor that does thorough quality control of their parts and really stands behind them if you ever have any issues.

When you see stuff on Amazon or eBay, you are generally seeing the bottom of the barrel parts. These are the parts that pretty much every other major vendor has refused. Vendors do random checks of available lots and if the defect rate is higher than they'd like to see, they will pass on the lot. It doesn't mean they are all bad though and doesn't mean there aren't any good parts in those lots.

So yes, you could save money by buying on Amazon. If you have a tight budget or don't want to spend too much on an old phone, by all means order those parts. But there is a very real probability that you will receive a crap part and if you do, you may have a hard time getting an exchange or refund.