Interacting and rumbling are good options. Make sure to get the
amulet coin from the shop asap so you gain more PD from
interacting. Remember to use your GGC as well, this will net you
some items to sell, too.
Get more rumble bags, level up the rumble areas, and you'll get
more PD and items. Boxes and keys are particularly nice to either
open for good items or sell off.
As you do more in the game, other options like shiny hunting will
open up to you. I don't battle but to my knowledge, battling
doesn't directly give you PD. You can spend the battle points to
buy items to sell, though.
Like they said, rumbling usually gives you mystery boxes that you
can sell off, interacting with other user's Pokemon, and using the
tool shed to make stuff to sell as well.
A PokeForumer, since 2019.
Send missing plushies, also buying missing dex mons
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