I have been testing this for a while and it works… so I wish to share it with all of you…
just to keep it low though otherwise the developer might banned this method again.
first of you should have a friend whom you can trust to trade with otherwise it won’t work
so here’s how it’s gonna work:
buy a mystery box – open it – don’t like it – then press refresh (F5)
by luck sometime this mystery box will remain unopened after you refresh it will be a mystery box still sitting inside the chest. And if you open it again it will give you that same item that you dislike so you have to gift that mystery box to that someone you trusted and that person upon will receive this gift in their doorstep. That person can open it an if dislike the item press refresh again and it will be in front of their doorstep again and can open it again and will render a new item.
so you can trade mystery boxes with trusted friends and it will make sure you get the item you want
some items from GMB may have big resale value such as 999 coins so good luck on that
for item references please refer to: pet society database.