GTS is very simplistic in how you can request Pokemon, the only things that you can specifically restrict are:
- Pokemon Gender (if the Pokemon you are requesting has one)
- Pokemon Level (you can request that a Pokemon be within a certain 10-level range, but not 'Levels > 50' or 'Level 100 only' etc.
Thus stuff like Individual Values (IVs), Natures, Abilities, whether it has the 6th-Generation Pentagon and Shiny values cannot be filtered on. You can use the description field to request these specifically, however it can be ignored by the trader. There is no 'take-backsies', and no recourse should someone trade you something you don't want.
In summary, what this means is that -should you put a Pokemon up on GTS- you risk getting traded a dud for your hard-earned Pokemon.
So what can you do?
- Only trade rare Pokemon for rare Pokemon - Request legendaries or failing that, higher leveled Pokemon. Generally speaking, if a Pokemon is level 91-100, then the trader has had to put effort into training them (cheating notwithstanding), and thus you're more guaranteed to get a better Pokemon. It's still not a guarantee though.
- Only trade easy Pokemon for easy Pokemon - have a 4IV Protean Froakie, but need a Gale Wings Talonflame for your own breeding efforts? Offer away! You might not get what you want first try, but if you don't, no biggie, there's plenty more where that came from!
Of course, when trading shinies, I would recommend:
- Avoid GTS Entirely - Join Trading groups online, you can find them on most forums, on Reddit, Facebook and other social platforms. Specifically request what you want, and what you have to offer in return. The best part of this is - the trading is manual, so if you don't like what someone is offering, don't accept and walk away (or if the other person gets rude, report them because that sort of behavior is wrong).