Did you ever knew about this price dealing chat?

Yep

kinda ya

Wow I never knew that xD

Ye, but it's kinda useless with that price limit.

Ye I knew it

I play this game long time and never knew this :DDD

It basically let's you Negotiate with your seller/buyer to find a decent price range suitable to you and your customer :D I don't really use it though because I'm only in Private Shop during item drop events. (Pearls, Coins, Plant food/Fertilizer etc...)

holy mother of a glob how do you do that?

When you open a private shop, there will be an option that lets you talk to the seller, if they are not AFK...