After purchasing be sure to send a message with your Friend code and In game name, and specify the Pokemon you want with all their details.
Things that can be customized are: Pokemon, IVS, EVS, Moves, Ability, Nickname, Pokeball, Item, Nature, Shiny, OT, Gender, Pokerus, Ribbons, and Level.
They must be things the Pokemon can legally have in the game, or they will not be able to be traded.
Disclaimer: All rights to Pokemon belong to Nintendo and Gamefreak.