Collect 8 Ghost-o-plasms.
There's a valley to the southeast, the Valley of Bones, haunted by Magram ghosts. Sounds scary, doesn't it? Well think how scary it is to the Magram! If we captured ghost energy from that place, I bet it'd be worth plenty to those centaurs.
Here, take this crate of ghost magnets. If you place one between the two big skeletons in the valley, the Dead Goliaths, then ghosts will wander to it. Distance yourself, take out the ghosts and get their ghost-o-plasm--that's what we'll sell to the Magram.