I'm not for this, personally. I think for things that have various "tiers" to them (Food and Munitions currently) we should only be able to purchase the most basic "tier" (Bananas and Cannonballs in this case). This encourages still doing some supply hunting, which is a core part of the game, in order to net a few higher "tier" items alongside the basics.
With that in mind, if we ever see an expansion to the Maintenance portion of things, if Planks don't end up being the bottom "tier" in that anymore I would say whatever ends up being that low end should be the purchasable option (though, for simplicity, if this is ever expanded I would think they would make the existing item the low "tier" much like they did with Bananas when they expanded the Food and Cannonballs when they expanded Munitions).
That is my personal sentiment on the matter. However, that said, I would be open to allowing Pirates to purchase Bait (though I would do this at the Seaposts with the Hunters Call) as well as Collectors Chests (and these I would have picked up from the Gold Hoarders) with the same limits as seen on the existing Supplies. I might even consider having Tridents available (from the Order of Souls in this case) to give all the Factions various luxuries you can purchase that are sort of themed around the respective Factions and what they might have an interest in selling (you could make these sellable too, for significantly less than they cost to purchase - as should be the way for all the purchasable things, they are an investment for speeding up various things you could just do otherwise). Hell, would could even do the most basic Rowboat from the Shipwright too.
Again, just my two cents on the matter.