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 :)
HORSEISLE ANSWER:SpiritEverPuppy-Brown Server
There is no actual way to hack into HorseIsle.
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.