Building a Raised garden Bed/Planter box
For any area that you will be growing food, pressure treated wood is not a good idea.
Batu decking is a kiln dried natural wood product. It is not treated with preservative copper or arsenic, and thus is a safe product to use for your purpose. It will eventually turn grey and begin to rot but will have a longer life than most wood. Although expensive, since your decking is unused surplus I imagine that this is as good of a use for it as any.
You will want the planter box to drain. Lining the box will not prolong the life of the wood, but would keep the water on plant roots and rot them.
Sealers will introduce chemicals to your garden soil.
I would just build the box and enjoy it for a few years. You may be surprised how long it lasts.
I hope this helps,