Undefined Reference to Function error?

I have seen other similar questions being asked but I could not figure out what the problem is. I have a declaration in an inventory class as:

class Inventory
{
   Public:
      void print();
      void sell(Item*);
      void add();
      void find(string);
   Private:
      Item* first;
}

And then in the inventory.cpp I have:

 void sell(Item* item_name)
 {
    ..........................
 }

And the error comes from calling it in main() as:

Inventory store_inventory;
Item* cur_item;
cout<<"Item name: ";
string name;
cin>>name;
cur_item = find(name); //find returns Item*

store_inventory.sell(cur_item);

The error is one the line for the call to sell. Any ideas?