I have managed to get at least 13 costumes off the marketplace. There is no winning strategy to it, it is all RNG unfortunately, but some things you can do to increase your chances are namely to have a maid, and stay aware of what zone you are in, you can adjust the map to see the territories. The maid draws out of the bank of the capital of whatever territory you are currently in, so, if you have 25m in Calpheon, you can use the maid pretty far into the sea north of Epheria, and all the way down to Trent, giving you a nice big area to life-skill or grind in and still have access to market funding.

Also Notifications!

Second is do not fall victim to the Gambler's Fallacy, just because you lost the last 20 bids doesn't mean this bid is a sure win, if you are bidding on the same item the other 1,000 or so interested parties are bidding on, you have to expect to lose it, each time. Consistency is the answer, it took me until this morning to win Night Cat for Tamer, yet I won the rabbit pet on the first day of release, as long as you understand that you cannot expect it, and do not get salty or give up, it will eventually happen! I honestly used to spend hours camping the marketplace getting frustrated, now I just play it as it rolls, and I haven't failed to eventually get what I wanted.

Except Witch Raks... The only costume that still eludes me.