Virtual item trading

1. Free-to-Play vs Pay-to-Play (Player-Centric Gamification Tips)
2. Microtransactions vs Macrotransactions (Virtual Economy Gamification Tips)
3. Game Progression vs Game Advancement (Gamification Achievement Systems Tips)
4. Player Challenges vs Player Objectives (Gamification Player Progression Tips)
5. Virtual Auction vs In-Game Auction (Virtual Economy Gamification Tips)