Bard is a AP support carry, buying pure armor/magic resist items is useless because you lose damage and you don't gain much HP from them so you're still very vulnerable to burst. Never buy Ardent Censer because there is a cheaper and better alternative - Unholy Grail, which Bard makes out of its passive more than Ardent's, don't buy Twin Shadow because it's not as effective when you don't have Glacial Augment. If you want to be relevant in the game you at least need to buy one AP item so you can do damage in early game whenever you're the strongest, buying first item redemption or locket makes you very weak.

Liandry's - Broken item, gives free AP, also HP so you don't get bursted as fast, always get it as a 4th item if the enemy team doesn't have any threats and you're looking too snowball the game.

Zhonya - Get this item first if the enemy team has huge burst potential or get it as a 4th item if the enemies are full AD.

Banshee's - Rare situational item, but very effective if enemies have a lot of CC and you're looking to catch someone out with your Q.