1) Obtain "perfect" amounts of all missable items (eventually).
2) Obtain maximum amounts of all missable unique items.
3) Open all missable chests.
4) Search all missable field icons.
5) Obtain the Athlete Queen.
6) Complete a Level 1 Game.
7) Deliver all the Moogle Letters.
8) Buy all of Stiltzkin's deals.
9) Complete the Mr. Morrid's Coffee sidequest.
10) Defeat all missable Treno Weapon Shop monsters.
11) Obtain the Moogle Suit and Autograph.
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1) OBTAIN "PERFECT" AMOUNTS OF ALL MISSABLE ITEMS:
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There are 31 items in the game which can no longer be bought on Disc 4.
Furthermore, 9 of these items are used in synthesis, which makes another 8
items potentially missable. 2 of these items are then used in further
synthesis combinations, which makes another 2 items potentially missable, for
a grand total of 41 missable non-unique items in FFIX.

The challenge here rests on having the funds to buy all these items, and in
buying them in the correct amounts. Although it may be fine to have only 1
copy left of each of these items at the end after synthesis, I have decided on
a different, more "perfect" amount. This guide will have you buy these items
in such amounts so that you have enough of each of them to equip everyone who
can equip them, plus 1 remaining for your inventory. This also includes the
items which are synthesized from these missable items, which is why some of
them are bought in larger quantities than one would first think are necessary.
In this way, these missable items will never be able to leave your inventory
unless you sell them.