Cosmetic issue. It's normal for buffers to get scratched. Buffers get scratched up when you shoot a lot. If it really bugs you, you could try to smooth out the rear of the carrier, there could be a raised spot.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek