Does anyone know of a trait which could aid a trapper ranger?
There is the elaborate trapper trait. But that's only for kobolds.
If it's a PC, I'd actually recommend Dangerously Curious, that makes Use Magic Device a class skill, along with a +1.
A wand of mage hand could set off an area trap the baddies coincidentally missed, or saw. A wand of invisibility would make the perception check DC impossible for a few minutes. A wand of grease could make baddies on a slope slide down into your pit trap (tho that'd be a dm call for the slide, course).
A few traps do require scrolls as well.
Well I have actually made a human snaresetter rogue with racial heritage: kobold. I picked up pragmatic activator and pretty much maxed intelligence. He is pretty sorry in terms of combat but I figure a well placed poison trap doing 1d2+2d6 con damage can count as pulling my weight. what's really nice about the poison trap is that it looks to me like the (Su.) is without a save. The traps I have picked up are: tripwire(free), poison, spell storing, selective, and swarm... I was debating smoke or snare over swarm though. He has heart of the fields to give him an addition ½ character level to craft:trap.