If the PCs kill the efreet in Darl's group, won't he return to his home plane, along with all his carefully-described possessions?
Depends. Is he Called or Summoned?
Sorry, I don't own the magazine, but here's some info:
Calling: A calling spell transports a creature from another plane to the plane you are on. The spell grants the creature the one-time ability to return to its plane of origin, although the spell may limit the circumstances under which this is possible. Creatures who are called actually die when they are killed; they do not disappear and reform, as do those brought by a summoning spell (see below). The duration of a calling spell is instantaneous, which means that the called creature can't be dispelled.
Summoning: A summoning spell instantly brings a creature or object to a place you designate. When the spell ends or is dispelled, a summoned creature is instantly sent back to where it came from, but a summoned object is not sent back unless the spell description specifically indicates this. A summoned creature also goes away if it is killed or if its hit points drop to 0 or lower, but it is not really dead. It takes 24 hours for the creature to reform, during which time it can't be summoned again.
When the spell that summoned a creature ends and the creature disappears, all the spells it has cast expire. A summoned creature cannot use any innate summoning abilities it may have.
Note that any items given to the efreet after coming to the Material Plane will be dropped in either case. If it looks like the PCs are gonna be cheated out of their choice loot, edit it so the items he has are given him by Darl.