I've been playing since the end of April 2015. I have most units leveled that I use or are likely to use (But only 2 AW with 2-3 more ready). Only 3 gold units used regularly, 3 more occasionally, and 15 I never use. The ones I never use are lvl 50 but not CCed. Being stuck at the "Wall of Magic" afforded me far too many gold spirits to use on just gold units and I don't use silver+ for fodder unless they are the same unit and when I do I use the appropriate spirit for 2x-8x exp so even then the gold spirits are not useful.

The closest I will get to selling silver units is "trading" them for the units in the trading post (Which I did to get Niel,Ertel and fully CR Percis & Khuri) and the bronze fodder I currently have will be used to quickly lvl Mikoto once I AW her.

If this is how you feel I probably shouldn't tell you about the time I sold 20 black spirits before I had my second barracks. My only black at the time was Saki and she was already leveled, Even now my only other black is Mikoto and she was fully leveled instantly and ready for AW and will be rapidly leveled after that as well(Hopfully next week, this week I couldn't get the last orb no matter how much I tried, damn you RNGesus)