When trading unopened booster packs, it does NOT MATTER (as far as Ive seen) which packs you trade
The ideal example is Leyline of Sanctity it costs zero mana, so if we have it, we wont lose any tempo at all and will be able to deploy our threats while being protected from various graveyard hate options
It sorely lacks trample, but green decks come by it often enough with cards like Garruk's Uprising that we can forgive it
It changes momentum instantly, commands respect in competitive play, and holds collector prestige that puts it in a class of its own
Unfortunately, my Liesa deck is the only deck of mine that can gain life reliably enough to justify the inclusion of Well of Lost Dreams, and I definitely like Idol of Oblivion, but the only deck of mine in which that card would be reliable is my Ghired, Conclave Exile EDH deck, and I am not certain what card I would remove from that deck, to make room for the idol
But thats to be expected, isnt it