If you have a guild bank with say 2 or 3 tabs...
That can serve as your warehouse.

There are a ton of items which sell very well on weekdays (raid flasks for example), but are generally flooding the market on weekends. Buy them in bulk on the weekend and just stick them into the warehouse. During the week, make sure you have several listed by early evening each day, average price on raid days will be quite a bit higher then average price on weekends.

You can extend the same principle to other items, not just flasks.