It is the most relieable way to transfer those Pokemon over to your X game and fairly easy.
Just go under "What Pokemon?" and see if the ones you want are available. Warning: The people can and will rip you off for an expensive price.
Not at all relieable because most would want to keep their legendaries.
Trade with Friends!
The most relieable way and with friends you can usually get them for a cheap price :P (most of the time depends on the trader)