Well, the fickle finger of finances threw me a curve. After two weeks on Gunbroker, no one bit on the M4, even at $650. However, someone gave me $700 for the retro AR15 as a parts kit, less stripped lower receiver! As much as I wanted to keep the retro, I sold it (less receiver) and put the M4 back into service as my patrol carbine:


Now I have a receiver for a .300 Blackout build when finances improve!