A Landowner Agreement is essentially permission granted by the landowner for caches to be placed on their land, provided the placement meets the landowner's requirements. The permission and conditions apply to anyone wishing to place a cache on that land regardless of how the cacher may choose to advertise the cache. As such, I feel that they should be hosted by GAGB rather than by any listing site.
Do not go gentle into that good night.
Rage, rage against the dying of the light. (Dylan Thomas)