I agree with Stakatacool, I've been resetting to recapture legendaries multiple times to try to complete the national dex, but for competitive then I probably wouldn't do so. It is a pain to collect bottle caps and I've noticed that in the GTS that there is a kind of value system where if you offer a certain pokemon, then you should ask for a different pokemon of same or less value, there is no true proffit in the GTS. (An example would be that suicune would be more valued then Raikou just because it is a trio master). Values can go in order(top most valued by others).
- trio masters
- non trio masters
- Sub legendary trio masters (Landorus as an ex.)
- Others in sub legend non trio
Also if you have a shiny pokemon than that will almost guarantee you any pokemon in the GTS.
So you can get the legendaries you want by trading in another legendary of or more value.
But if you do have a second game then I would recommend trading the Items there first then resetting.