There are many quests throughout the game that you will find that require you to obtain items that could be easily bought elsewhere. If you knew about these quests beforehand, you could just keep the item in your inventory for when you needed it.
Here's a list of buyable items that I know of. This is far from a complete list but if you add items to it I'll be sure to keep it updated.
Shredder Operating Manual (Pages 1-12) for the Shredder Operating Manual quest
Green Hills of Stranglethorn (Look on Thottbot for a complete listing)
Frost Oil, A Gyrochromaton, Healing Potion, Lesser Invisibility Potion, and Patterned Bronze Bracers for the Kargath Rock Elementals quests.
15 Wasteland Water Pouches for a quest in Tanaris.
A stack of Silk Cloth for the guy in the outhouse in The Searing Gorge.
25 Un'Goro soil for a quest in Thunder Bluff, and one more in Tanaris.
All the body parts of animals you need for the Blasted Lands item buffs, as well as the imperfect and perfect dranethyst crystals.
4 Abyssal Signets, 1 Large Brilliant Shard, and 10 Encrypted Twilight Texts for the Cenarion Hold quests.
30 Crypt Fiend Parts, Savage Flora, Dark Iron Scraps, Core of Elements, and Bone Fragments for all of those quests at LHC.
15 stacks of Wool, Silk, Mageweave, and Runecloth for cloth turn in's to capital cities. At level 53 this will yield somewhere around 80K XP
Pair all of these with prepared leveling gear and potions, and follow a good guide, and it really wouldn't be hard at all to level.
PLEASE TELL ME OF ONES THAT I MIGHT HAVE MISSED. IF I GET ENOUGH OF A RESPONSE I WILL CLEAN IT UP A LOT!!!