so .. starter pokemon are loving gold mines dude.
I traded a charmander level 1 for an abomasnow .. and a bulbasaur level 1 for a haxorus level 67. you can get pretty much anything you want... and then you just breed the starter for unlimited of whatever you want ..
I have all 3 x/y starters and gen I starters ..
also, sylveon is a hot trade pick. so many people want one. ampharos, too. and ampharos are so easy to get, just level a mareep from route 12. you can get such better pokemon for no effort at all.
also also DON'T BE ONE OF THOSE loving PEOPLE WHO PUTS A QWILFISH UP EXPECTING AN YVELTAL. you can go to any loving pokemon on the GTS and there are idiots asking for legendaries. why??? you're taking up a search slot and wasting everyone's time
PRO TIPS:
yveltal and xerneas' only worth are in trading each other. every copy of X has xerneas and every copy of Y has yveltal. they are not worth anything other than to trade to each other.
zygarde is worth NOTHING. do not trade your zygarde away asking for a poke. why? because EVERY copy of the game has one. this exact same thing goes for mewtwo.
check unwanted pokes on the GTS frequently. you will find that some of them have items ... and you can get them very cheap. I got an avalugg holding a king's rock, for some low level poke I didn't need. such a good deal.