The brass nipples are much more sellable than the aluminum ones.
Aluminum nipples belong mostly to the weight weenie world and has some compromises that most won't want to live with on their wheels.
I always traded in aluminum nipples that came with some spokes sets I've bought, straight for brass DT nipples when I picked them up from the bike shops I ordered them from.
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72 Line Seeker
83 Davidson Signature
84 Peugeot PSV
84 Peugeot PY10FC
84 Gitane Tour de France.
85 Vitus Plus Carbone 7
86 ALAN Record Carbonio
86 Medici Aerodynamic (Project)
88 Pinarello Montello
89 Bottecchia Professional Chorus SL
95 Trek 5500 OCLV (Project)