Gameplay and Story Integration: As the person most experienced with this place, he helps to keep the madness at bay, not only helping getting rid of Tekeli-li cards, but also managing mental traumas (something normally impossible)
Chances are good that at least a couple of them appealed to your deckbuilding interests, but with a volume this large, its entirely possible you might have overlooked some cool ones along the way
You also need to double check that the cards are playable in tournaments as some people sell cards which can not be played
The dilemma puts your opponent up against two clocks at once: Either face down the inevitability of an extra card each turn, or the eventuality that the owner mills a chunk of expensive cards and kills you outright
Special variants like serialized cards, old Reserved List prints, and limited promotional cards can be even rarer due to extremely low print runs.