4th Gen Is it possible to complete the Sinnoh dex without Wifi?

[Far Way]

Because of the Wifi shutdown and me having no interest in Wiimmfi DS or AR, I want to know if it is possible to see (not catch) the first 150 Pokemon in the Sinnoh Pokedex without trading or GTS.

 

[PurpleOrange]

Because of the Wifi shutdown and me having no interest in Wiimmfi DS or AR, I want to know if it is possible to see (not catch) the first 150 Pokemon in the Sinnoh Pokedex without trading or GTS.

you have to see the first 150 (d/p) or 210 (pt) in the sinnoh pokedex to get the national dex, so yeah you can

 

[Rosehip]

Because of the Wifi shutdown and me having no interest in Wiimmfi DS or AR, I want to know if it is possible to see (not catch) the first 150 Pokemon in the Sinnoh Pokedex without trading or GTS.

Very possible. Whether it's in Diamond, Pearl or Platinum, you can see all the Pokemon in the region without even knowing what the GTS is. For the Pokemon you can't catch (like Manaphy, for example), you can usually find a place to "see" them in a way, where you can't catch them but you have their data in your Dex marked as "seen". You can find Manaphy's info in the Pokemon Mansion if you're playing Platinum, to put more into my previous example.

Edit: I completely ignored D/P, sorry! As for Diamond/Pearl, I believe you can see the opposite legendary to the version you're playing when in Celestic Town, I think. Correct me if I'm wrong, anybody.

 

[Astinus]

Edit: I completely ignored D/P, sorry! As for Diamond/Pearl, I believe you can see the opposite legendary to the version you're playing when in Celestic Town, I think. Correct me if I'm wrong, anybody.

That's correct!

Just for anybody looking for that information:

You can find the old lady in the large house on the north side of [Celestic] town. Talk to her, and she will show you the picture, counting toward seeing the Pokemon for the sake of your Pokedex. She'll actually show you this picture whenever you talk to her after you see either Dialga or Palkia at the Spear Pillar