Personally.....And THIS IS NOT CONFIRMED AT ALL.....

I believe this increases the chances of a rare item

Step #1 ~ Poach untill you get the rare item you want.

Step #2 ~ Do NOT BUY it from the Fur Shop

Step #3 ~ Go out and Poach and Poach and Poach

Step #4 ~ When you go back to the Shop (It seems to me that) More than the Normal amount will have become the rare.

I believe that not buying the rare items increases the chances of Getting that Item.