In Pokémon Crystal, there is a Trainer in Olivine Lighthouse who uses a Politoed in rematches. He is the only trainer who has a Politoed in all of Gen 2. This means that the only trade evolutions that can't be registered in the Dex with just one system are Scizor and Porygon2. All others can be encountered at least once:
>Alakazam
Sabrina (Saffron City)
>Machamp
Bruno (Indigo Plateau)
>Golem
Hiker Michael (Route 45)
>Gengar
Morty (Ecruteak City)
>Politoed
Sailor Huey (Olivine Lighthouse) (Rematch)
>Slowking
Pokéfan Alex (Route 13)
>Steelix
Jasmine (Olivine City)
>Kingdra
Clair (Blackthorn City)