Shiny Pokemon must have proof that they are under your OT/ID.
From the Rules:
Due to the ability to modify code to make Pokemon shiny in X/Y (also being unable to tell the difference), I'm tightening the rules a bit. Selling shiny Pokemon is NOT allowed here unless :
1. You hatched them under your game's OT & ID.
2. You can provide proof to the Reddit thread where you got the egg hatched.
3. It's a Kalos event such as the shiny Jirachi with moonblast.
Anyone found selling hacked shiny Pokemon will have their Play Coins disabled. If someone messages you in private to sell a hacked shiny Pokemon, please report them and (1) their Play Coins may be disabled (2) You'll get all of their Play Coins.
#2 depends on if they're aware of this rule and still choose to sell hacked shiny Pokemon.
The selling of perfect IV non-shiny/shiny Ditto clones are allowed for personal use ONLY.