You buy used phones, you can more or less expect them to be repaired. If you want the guarantee of a 'fresh' phone then you should pay the mark-up. Even the seller might not be aware that the phone has been repaired.
Why? If a repair costs $x with genuine parts and 1/3 of $x with aftermarket parts and the phone works then I really don't see any good reason. Cars have aftermarket pars fit to them all the time, brakes, tires, shocks, filters, engine components (turbos!) and so on. They're arguably a lot more critical than the screen of a phone which is just another wear part.