For example this enchant : Eternal Chest
For this enchant you need 1 Eternal Crystal and 2 Sacred Shards
To create an Eternal Crystal you need to disenchant a purple (Epic) item. For example you can see in the picture that Eternal Stats is sold in the AH for 31G. But most purples can be sold for more gold , in the example, I got a purple neck that can be sold for 37G with any NPC. So, it is more profitable to sell this purple instead of disenchant it, use 2 sacred shards and sell it on the AH. No mention that there are purples that can be sold for even more gold.
Unless I am missing a very fast and cheap way to farm Eternal Crystal, what is the point in selling enchants on the AH?