In any shop, you can sell items in stacks of 100 to the shop under "Merchant Supplies." A maximum of 10 stacks per day are allowed.
Starting with the first stack, you unlock an achievement. After selling 30 stacks, you receive a "Packing Station I" as a reward.

You then have to set up this Packing Station I somewhere and can then also "sell" items there.
You only receive "Merchant Tokens" for selling items at Packing Station I.

Not, as previously assumed, by selling items to the shops.

Make sure you have a packing table to pack the fish