OP was referring to the uppers, but put lower in the title by accident.
As the ad says, they are functionally perfect. Cosmetic blem on these means one of the following things happened:
- Anodizing has lines or waves in it
- Upper was dinged in transport and has a scratch or other mark on the exterior
- Engraving was performed incorrectly (T-marks or logo misaligned, etc)
Once one of these issues was noticed on the part it was set aside and never had the ejection door or forward assist installed. The metal and dimensions are still within specs and is good to go. So if you don't care about your weapon looking flawless or winning beauty contests, then these are a good deal.
I don't know if they will, but if you place your order by phone, you may be able to get them to build out the upper receiver for you for just a little extra and still come out with a complete receiver at a great savings over a flawless example.