Two questions
 

Why do people want supers, if they end up trading them for supers, and those supers, they also trade¿

Why don't people like to trade trash, or accept trash when they do accept shells¿

1. People like variety and having rare items make them feel special.

2. Shells have become a sort of standard. Plus, you have only two prices... Compared to trash, which are almost all different prices.

1.- But they will just keep trading those non stop, making a cycle
2.- It's only $4, 5 and 6 while Shells are $3, 5 and 7 It's the same, and the range is better since you get more $6 on trash and on shells you get more $3