This item disappears if not purchased during Act 2.
Bonus to evade traps increases the chance that a character will avoid the damaging effect of a trap (by leaping backwards, away from danger). It does not allow a character to walk over traps without triggering them and losing the experience the trap could have provided. Its use is questionable for low dexterity characters and for high cunning characters. The former will almost never avoid trap damage even with the amulet equipped, while the latter will have no trouble detecting and disarming most traps.
There are only a few instances of scripted traps which cannot be disarmed, most of them have hidden shut off switches that can be detected and used instead.