The weapon shop was in Lindblum, not Treno.
Though that was my favourite as well. When I first played FFIX I had never played any FF before 7.
Kain's Lance(Kain the Dragoon from FF IV)
Ramuh's Story about Josef(Josef from FF II)
Magical Fingertip(Artifact that talks about Gogo from FF V and VI)
Garland, the 4 Elemental Fiends, and the Rat's tail(All from FF I)
Freya's Dragon ability "Reis's Wind"(Reis the Dragoneer from FFT)
The Cloud and Squall line(Cloud FF VII, Squall FF VIII
Garnet's real name(also from FF I)
Doga's artifact and Une's Mirror(FF III)
Others that I haven't mentioned or completely forgot about
The weapon shop was in Lindblum, not Treno.
Though that was my favourite as well. When I first played FFIX I had never played any FF before 7.