page 128: "Magic items are the DM's purview, so you decide how they fall into the party's possession. As an option, you can allow player characters to craft magic items."
What follows are all the necessary guidelines for that process, including costs, time required, prerequisites and rules for combining effort. It even directs you to the guidelines for creating new items.
Notice how I used "guidelines" instead of "rules" above? That is because that is what 5E is really working toward and why it is, IMO, the best version of the game to come out in a very long time. D&D is not like chess or Monopoly or even Warhammer (or Chainmail, for that matter). Because it is a game that ultimately centers around stories (some people plan those stories out, others allow those stories to emerge, and most everyone does something between the two extremes) D&D suffers when encumbered by endless hard coded rules. Even Gygax, who is often associated with a my-way-or-the-highway attitude, wrote explicitely in the 1E DMG that everything in the gamebooks was a suggestion and a guideline.
If you want tight item creation rules and rules for the adventuring economy, implement them. It is too bad that WotC has not introduced an OGL or similar license because it would make finding such rules to implement much easier. But even as it is, searching message boards and blogs will invariably turn up those rules -- and there is nothing saying whatever official rules WotC might have come up with would have been any better than what dedicated fans can produce. Moreover, those kinds of rules don't fit within the framework of 5E as it is written. This is a version of the game that goes back to interpretation and adjudication on the part of the DM in many cases, even spells and class abilities (and not to even mention the definition of "hidden" -- oops, I did mention it). That is the game it is. So your options are to tweak the game to get it the way you want it (huzzah, I say!) or to choose a game that better fits what you are looking for (huzzah, I say, also!).
What really is not a viable option is grousing about how the design team failed because they put in a system that meshes absolutely perfectly with their design intent.