If you just happen to have the materials, you're still better off selling them and buying the crafted item. It doesn't matter whether you farmed for the mats or just picked them up casually, you still have to factor in their value on the TP to see if it is more profitable to sell them or use them.
^this, is what I'm trying to say to everyone who makes that mistake...
You're better off just buying them. There is hardly any mark up. The only way it becomes a good profit is if you farm the materials yourself then craft them
This is a misconception, your farming time and total farmed items =/= free stuff
In fact, most of the time it will be even more profitable for you to sell your farmed items and place a buy order on the item you wanted to craft