During your untap phase, you may tap this card to trade with a Methuselah who agrees to trade. You may trade pool and cards from your hand and cards you control. The trade cannot result in either Methuselah have fewer cards in hand than his or her hand size. Any additional terms can be established, but none are enforced by the rules.
Clarifications and Rulings
You can make as many trades during each untap phase as you like. [RTR 19970630]
Non-equipment, non-retainer, minion cards (like Ablative skin) can be traded, but will remain on the original minion. Control of the card changes. Mostly this results in: "no effect" (until the controller is ousted, of course). [LSJ 19980128]
Trades cannot result in a player needing to draw cards (card text). So if player A trades a Elder Library to player B, player A must do something (like also trade away a card from her hand) such that player B doesn't end up needing to draw to fill her newly-increased hand size. A player who begins the trade with fewer cards than hand size (because of an empty library) is OK, since the trade will then not "result" in the problem. [LSJ 20010907]