You need to be at least Level 100 and can only trade/sell item with rank 35 and above, or equips level 70 and above.
Happens rarely. If its happen, you can see it at Daily Activities tab in game.
Nope, you can't. Do remote training with EXP potion! You can win free training island voucher (1hr x3) from daily Octopus!
Yes, if you fight monsters that are 19+ of your current Level. Anything lower or 19 of your current level will not break your equips.