The Amount can only be set to 100. 30000 would be better, if one wants to exchange downward, i.e. into Copper.
If you buy Coppers and you have Coppers in your Inventory you pay the Coppers with Coppers, so nothing realy happenes.
Secondly she cheats
. If you only have silver and buy 1 Copper you pay 1 Silver for 1 Copper.