I am, and will always, be against the purchasing of supplies. Not only does it, literally unavoidably, gives a benefit to 'wealthy' players over newer players, but it detracts from the management of the ship, which is a really important aspect of sailing. A crew that can manage it's resources has a benefit over those that can't/don't. Allowing supplies to be purchased completely removes this ability-based facet from the game. Restocking on the go is not difficult at all and basically removes the need to stop and 'farm' supplies. Essentially, some of the best practices you can do are:
The Golden Rule: unless combat is imminent, all crew members (except the dedicated helmsmen, if there is one, since they never leave the ship) always keep an empty inventory (no cannonballs, planks, or food at any rate)
While traveling:
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always keep someone looking out for barrels. Harpooning barrels can be hard, but the restock potential is very high. Even if you miss, people (up to half the crew) can still jump off for the barrels and then mermaid back.
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every time you pass an island you're not going otherwise stop stop at, have half the crew jump off and pick up as much supplies as possible, then mermaid back. This and harpooning barrels are the 'passive' way to restock and can generally stock everything you need unless you're getting constant back-to-back action.
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hit every sunken ship you can. These are often crammed with supplies and it takes much, much less time to drain these of resources than it does an island, by virtue of them being more compact (less traveling time). Having a rowboat makes this even faster
If you're a PvPer: siphon other ships. e.g. zone them out into cannon/lunge boarding range, board, anchor, kill the crew, and then cart resources from their ship onto yours. This is best done when you have a surplus of wood and a deficit of cannonballs, since no shooting is strictly required and you can also afford to get shot at. Either ship having a rowboat makes siphoning dramatically faster.
Note: if they're doing a tall tale, be a nice person and don't sink them. Still steal all their supplies, but let them keep the ship and the tall tale items. Sinking them basically resets their progress. Almost all siphoned ships won't try to steal their supplies back, by virtue of not being able to (no supplies).
Note #2: some PvE only crews will literally let you take their supplies in order to avoid a fight. This is the holy grail of honorable PvPers ("honorable pirates" do not mean 'kind pirates'; it means not causing suffering beyond what's necessary for their goals) trying to restock, since it's still player interaction but you're not mashing down on people who actively don't want to fight.
If doing a quest: always dedicate one person to stocking supplies while others work on the quests. Rarely ever does a quest require every crew member working on it. Swap it out each island if people aren't gung-ho about that role.
The "Big Addendum" to this is that I may be fine with the exchange of treasure for supplies. e.g. every outpost has an NPC you can bring any treasure to in exchange for a given amount of supply, or maybe session-based (a.k.a non persistent) currency that can be spent on supplies. Some system like that could be feasible, since it gives the advantage to players who have being playing longer within a single session, rather than players that have been playing longer overall. The feasibility of a system like this would really come down to the details, but all-in-all I feel it's best simply to avoid the concept of purchasable supplies altogether
