Trading Post, Selling and Buy Order listings
Why not just remove the 250 limit for order quantity ?
That way, you can easily buy 503 pieces of ore in one order, while your suggestion would still require two orders. One for 2*250, the other for 3.
Removing the 250 limit would work too, similar to what they did for the crafting amount.
Previously it was capped at 999 (3digit), now it is possible to craft up to 5 digits