Hi Sky

"Best" is subjective, and there lies the problem. As others have pointed out, one can buy a level 21 item with level 21 stats, but it's not so much more superior to level 20 items that it necessarily makes a difference, especially in a team environment. Yes, technically it's "the best", but does it matter? Even the new 2h aren't necessarily "better" than dual wield 1hs that are already in game. So, nobody is really forced to pay real money to have the "best" items, at least in terms of item usefulness. That being said, one is forced to pay for the "best looking" items, which is perfect for F2P game monetization.

You're correct in that trading cards can be trade or sold in the real world, however, in-game card packs, lockboxes, chests, etc... (depending on the game) were literally designed after the trading card pack model. The reason you can't sell the items is because it leads to corruption of in-game economies, black markets and ways to convert gold to real money.