This thread is still going? ;_;
The OP is taking the risk (albeit however small) to make the 300 gil profit margin per unit (30k per 100.) If the 'street price' is actually 5.7k per single on that server, there is no 100% guarantee that the OP will be able to find demand at 600k-per-100 and achieve that projected margin.
If the demand decreases for whatever reason on that server, the OP has a chance to get stuck with currency that cannot be moved quickly even at 600k-per-100 no? At that point, the OP has to either adjust for the market and mark-down (and decrease profit) or wait out until demand increases (delay profit.) So the risk is his, and the profit comes from both this risk and the convenience factor of his actions, I would think.
If the 'ethics' issue is instead that the OP is not actually using the coins to make a relic, and is thus a competitive consumer for a finite resource (and is only looking to make a profit) is that significantly different than those suppliers of those coins that farm them and sell them at-or-above-street-price (and are only interested in profit and not others' completed relic goals?) :/
In any event, I've admittedly bought stuff out of bazaar before that were priced super-inexpensively, purely with the intention of either selling it on AH or to NPC. If the supplier doesn't want it, and is ok with the price they set (and it wasn't some mistake) and others haven't moved yet on the opportunity, I personally don't see who is being wronged in that transaction. :/