As of patch 1.0.8, this achievement just got a lot easier. This update gave players the ability to discard voyages from their voyage inventory.
Not only that, but when you discard those voyages, the voyages sold by the Trader are restocked! So now you can just buy 3 voyages from the Trader, discard them, buy them again, rinse and repeat. Do this until you reach 250 purchases, and voila, achievement unlocked!
It is highly recommended that you grind this achievement before purchasing ANY promotions, as the voyages are free at the lowest rank. I have tested this strategy. I was able to "purchase" 250 merchant voyages at the lowest rank in less than an hour, and went from level 1 to level 9 in the Merchant Alliance just because of the commendations obtained on the way. Just be very careful to not accidentally buy a promotion while you are spamming the
I'm not sure if the Traders restocking their voyages is a glitch or not, so take advantage of this while you still can! If anyone can confirm whether or not this is a glitch, please let me know in the comments.