I just got d2r on the switch and I saw my characters when I logged in. Why would there be a d2r switch trading forum if you're able to log into your d2r accounts?

D2r is not cross platform, it’s cross progression.

Ie, your gear on account is on account regardless of which device you log in with.

However you can’t log in with your switch and join your friends pc game.

Hence separate trade forums.

So can I trade with friends on Pc and play with those items when I log in with switch?

Yes.