I trade professionally. i have over 1 and a half million shell value in items. now, your question:
the standard exchange rate for any sellable worth 5$ to a trochus is 1:1.2
the formula for converting a sanddollar (mushroom,broken bottles,brains,etc) is: (number of sandollars)/5 x 6 = converted to trochus
Now, the reason most of us dont just multiply by 1.2 instead is because when the original number is divided by 5, we get to see the profit we make exchanging.
Most traders will only accept shells because its easier to exchange those for trochus (less time, shell preference among widespan traders, etc)
I'll accept any form of currency sellable wise as long as its not flowers. But i will normally ask that it exchanges to atleast 200 more than standard super price with supers below 10k. and atleast 500 above price with supers worth 10k+
We trade our supers for shells, to buy other supers, not sell them for gralats. SO gralats play no hand in shell exchange rates it is an always will be 1:1.2... anything else is bs. plus, if youre particularly lazy, you could ask me to find you a specific super, make me an offer, and once i get that super (2 days tops), u buy it off me.