Great question! Here's a walk through our thought process:
When we make guides, there is a dual purpose of 1) providing free, good information to the public that empowers them to fix their own items, and 2) providing parts that correspond with these guides that can be purchased by a trusted company. Although we wish everything could be as free as the guides, we can't feed our families on thumbs up! :)
Having that said, it is important for us to offer parts that are trustworthy and tested (as opposed to many eBay offerings that are cheaper but are often horror stories by the end of the day). Most parts like the headphone jacks are really not test-able by us, due to the soldering necessity. Rather than offering parts that are untested and making guides for these parts, we prefer to sell parts that are tested as working, and have guides that reflect these parts (in this case, the iPod Touch logic boards with the jack attached).
This gives those who are new to this and a bit apprehensive about taking apart their machines the opportunity to repair their stuff with a skill level that they can handle, and those who are more experienced can still find the solder-necessary parts elsewhere to accomplish their repair!
Ultimately though, we desire for those who are experienced at these kind of repairs to make their own guides to post on iFixit, so all the information is freely available for all skill levels! This feature is rolling out piece by piece, and should be available in its entirety soon.