Noob question, can you trade with DLC people if you don't own the DLC

You should be able to trade with them so long as they still have their gear sitting on an expansion (pre-RotW) character.

If they're only running RotW/have that item on RotW then no, you can't trade that back into non-RotW.

Think of it like classic vs. expansion before RotW.
If both made a classic char, they can trade. If one is classic and one is expansion, they can't.
Same Principle now applies to classic vs. expansion vs. RotW, it just adds a 3rd "version" to the mix.

You cant trade with DLC character.

Some item will be 'cheaper' in old world.
The DLC player may still able to use LoD character to buy them and trade into RotW world.

As for the opposite, RotW back to LoD. You may need to pay some expense to change back.(and trouble, with player help with fee for reasonable no free lunch .)

As I know, many player and bots still in old world, that means some old items will cheaper in old world. The world will have different market soon...

Like, low runes and

Perfect Gems in new world is less valuable than old world, some even cant sell before new craft system show up.