Any Rock stores!
Dave McGee on Loch Isle buys them in tens, $500 for each batch.
Joylyn in treeton will buy your unwanted grasshoppers.
almost anyone with money, or who ever needs it.
MeadowTon Blacksmith
I am not sure if there is any places in caves where copper can be found. But one way to get it would be to have a ranch and put a copper mine on it for 20,000. This will produce 30 copper nuggets everyday.
On most isles, any isle with rocks! (I find Meadow Isle has a lot)
If your asking about General Stores, Plainton sells for the least and buys for the most. Tigerton sells cheap tack and that's all I know so far.
Depending on the classification of the metal a specific factory buys it! For example, copper scrap goes to recycling copper factories to be renewed, same goes with other materials :)
There is no actual way to hack into HorseIsle.
HORSEISLE ANSWER:SpiritEverPuppy-Brown Server
The US government (if you count production as buying them), coin collectors, and speculators on the value of copper and nickel (although defacing US currency is a criminal offense).
Horseisle is a computer game so it is international.