I'd recommend selling it after Free Trade has been released.
Reason why? I see you've got an offer of $1,100 (A/FREE TRADE), and $200 (GP+CASH).
Now look at the odds, you sell the Party Hat for 600M + $200, right? and then you sell the GP for $1/M, equaling $600; total wealth earned: $800.
That's $300 less then what you could of earned if you sold if after the 1st of February.