Is there any important missable item in Final Fantasy X?

Here's a list of all permanently missable items in the game.

• a no encounter weapon, as an equipment drop from Geosgaeno.
• a magic counter buckler, from the hovercraft merchant in the Calm Lands.
• the 0-Slotted Longsword can't be bought after a certain point.
• Master Spheres are limited in quantity.
• the four Al Bhed Primers from 19 till 22 are (technically) missable.

Also, certain non-items are also permanantly missable.

• Having Wantz in Macalania Woods selling 4 slot equipment
• You can't recruit Durren for your Blitz team anymore after a certain point.

For more information, there is a FAQ on GameFAQs giving detailed information for all of these items