At even higher levels that you use
L2 gold to arrive, items you enchant will be sealed, with the numbers in red when you mouse over them. In this state, they can be sold, traded, and so on, but that is not used. Equipping the item will give a prompt about breaking the seal. Once you do so, you can use it, but no longer sell, trade, and so on. What will be sure is that enchanting again to the next level will re-seal, but not about all cases. At the max enchantment of +10, you no longer have to worry about Durability. Also, there are currently limitations on max bonus. This is currently +4. If you are really serious about PVP, you will probably end up spending significantly more money to keep upgrading equipment of your teams to +4 of whatever your current level is equipment is. Still, do the numbers try and upgrade the items that give you the most effectiveness for your gold first.