Quick and probably stupid question...

[setstyle]

As long as you're dealing with raw materials or an item with full hit points, it can be stored on the merchant, but unfortunately you cannot re-buy anything from the mainland since Silan is regarded as a separate continent. However, bulk isn't as critical an issue once you get to the mainland and purchase mektoub packers, and you should readily be able to find crafters that can replace any of those rewards left behind.