I think however, you're over thinking how they should sink the items though.
There should be a buy back system with Torishop.
A certain percentage say 90% of currently listed market price, limited to about 50% or whatever of the original Torishop price. This way it would encourage the market price to be a bit higher and decrease inflation if people decide to "quicksell" their items back to the Torishop rather than waiting to go through the hassle of trying to sell that item on the market or forum and getting lowballed constantly by marketeers.

But yea, there would have to be some sort of cap to the amount of TC returned when the item is sunk.

OR

There could also be some sort of "trade up contract" implemented into the site where you can enter in a few items and get an item back (QI restriction taken away for the items given in return would be a good idea too). Sort of like how CS:GO trade ups work