The goblins have a different view regarding property than wizards. They believe this view works for them (we don't know enough to judge). Wizards depend on goblins, who have rebelled in the past. If wizards think they can demand that goblins abandon their beliefs and customs just because wizards said so they are preparing the ground for the next goblin-wizard war. Peaceful resolution can be reached if wizards take the time to think what goblins' beliefs mean to them in the context of goblin society (and even better if goblins do the same regarding to wizards). In any case, now that wizards are aware that ownership means different things to wizards and goblins, at least all future wizard-goblin deals should specify explicitly what fate should apply to the item in question upon death of the buyer.