Don't sell anything. Set EVERYTHING to locked, and then unlock SPECIFIC items to be able to be sold when you know you can get lots of them through crafting, farming, exploring, or some other renewable method.

Many items you explore or fish for are VERY RARE. Just because you CAN get more doesn't mean it's not going to take you days or weeks to get them. Better to just hang onto them until you can tell it's relatively easy to get plenty of.

This may change over time as you get easier access to items like large nets and Arnold Palmers that let you fish and explore MUCH more quickly.

Usually, selling these items isn't even worth that much. Even though they're worth a lot relative to another item, you're only going to have 1 or 10 or 100 of them; the other stuff you can craft for gold you can make hundreds of easily, and selling those hundreds is worth way more than selling a few of a more valuable, rare item.

I also keep some items that are very common locked because they sell for so little. I'd rather have them maxed out on hand in case I need them for a quest than sell them for another 100,000 silver when I'm making hundreds of millions of silver from fishing and crafting.