Somewhere right under "Sell My Things", or conversely, at the very end, or even next to Merrigan’s Exchange. A store that lets you also sell your things, for the exact same value as "Sell My Things" does… but with two differences.
First, it doesn’t list anything that can be bought in your other stores. So it ignores all the fancy clothing and expensive pets and stuff, focusing solely on trade goods (except, that is, you own more than one of a thing)
Second, and more importantly, it sorts items not by value, ascending, but descending, by total value. So that insanely huge stack you have of 10p items will come up earlier than that one fairly expensive item you have one of. Basically, a "what can I sell to get the biggest infusion of echoes right now" store.
Er. Please?