As Peter Kafka explained, writing for All Things Digital, the three companies have agreed to sell each other’s ‘Class 2 display’ inventory – graphic ads the companies can’t sell on their own and would normally hand over to ad networks. The firms gave a dinner presentation in Manhattan this week covering the plan. Attendees included prominent web publishers and ad buyers. The point of the presentation went beyond news of the new partnership. The three companies hope to talk some of the larger web firms into sharing their ad inventory, thus creating a larger consortium – something that might a…