I don't do much on this site but I wanted to share what I found that could possibly solve the problem that many Pokemon Diamond, Pearl, and Platinum players been having in the Underground that surprisingly had little to no answers in these past 15-16 years.
So, essentially, there has been an issue players had in the Underground where they're trying to give Goods to other players to get that elusive 'Pink Crystal' from Mr. Goods for gifting 100 goods, or maye just wanted to give a good that the other person didn't have, but every time they try to do so, they get this:
"Thanks for the offer, but I'll pass. See you again tomorrow."
After looking around for the past several days, and finding little to no answers, I was about to give up any hope of finding an answer... when suddenly, as of today, my Diamond and Pearl games started allowing me to give goods to each other, with no problem. (Some had stated this has happened with seemingly no explanation).
Well, now I believe I finally have an answer!
After testing it, I have found that the game has a penalty that people might not be aware of.
Essentially, you know how the game forces you to save before entering the Underground every time? Well, I think there's a reason for it.
Because if at any point, if you turn off your game while in the Underground, the game will penalize you in a way where the next time you go into the Underground, the game will not let you trade goods with other players period.
The phrase above even overrides the phrase you get if you try to trade something valuable (like Mr. Goods' items)
And it doesn't stop there. If you go back up aboveground, and you turn off your game without saving afterwards (essentially your last save being the one right before going undergound), the game will penalize you
Now, the solution is simple I believe. You just have to wait till the next day or so (24-48 hours), without doing the above mentioned things, and it should let you trade goods again.
I hadn't done much testing afterwards, but I do believe this is what causes that accursed phrase you get.
Also, random note: You cannot trade dolls. It doesn't matter if the doll is National Dex only, one of the kind, of something you can get from the start, you cannot trade them
TL;DR: Don't turn off your game in the Underground or without saving after leaving the Underground if you want to trade goods that aren't dolls