To buy 229 coke you need 572500 Grand Company Seals.

Let's say you're very lucky and get 29 as drops from dungeons, you'd still need 500000 GCS.
That's ~590 darklight items you have to deliver, or ~2500 FATEs.
It also means you fill your GCS cap 10 times, which takes some time, no matter how you look at it.

The question is, why would someone buy coke and store it, just to sell all of it at a later time for ~50% of what seems to be the normal price?
Makes no sense to me, but I'm always careful around retainers who sell large quantities of expensive/rare items.
Not saying it's not possible, but the chance of supporting a bot or RMT is high in such cases.