Recently, I've seen many Jam Mart items become rare. And I've never expected them to go rare at all. Just look at the Straw Hat, for example. Since when were those in style? I remember seeing those in Jam Mart not so long ago. Back then, nobody wore such an item, but now animals are lining up to get themselves a piece.
So what exactly happened?
No one can say exactly what happened, I have a theory. Because of the low demand in Straw Hats, no one bought them, resulting in low quantities of the item. I bet there are more Spike Collars than Straw Hats.
To get back to the topic, making an item rare is relatively simple. You just have to increase the value of the item. Let's just take a random item: the Soccer Ball Mask. Clearly, this mask is devastating when it comes to looks. There aren't even that many in Jamaa. Be sure to put this item on your trade list, and next time someone offers something, simply make them add. Even when offers get extremely tempting to accept, don't. This is crucial in getting people to think this item may not be in high demand, but that it is rare as a ruby. Even when Jammers are offering Headdresses or something, you cannot accept. This method deceives the person into thinking the item is worth more than the object they are offering.
It takes a team of people to create a rare item so suddenly. If even one person accepts an offer for their 'Soccer Ball Mask,' the item won't be able to reach the rare state.
Another way is to get another account and make them offer something rare for your Soccer Ball Mask. All you have to do is decline the rare item, and this will bring in attention. If enough Jammers are fooled, your item will become the next biggest thing.
Your item doesn't even have to be 'rare' or wanted to become rare. It just needs to have people that will believe in its rarity. And, nowadays, if the majority thinks that, it seems to become a fact.