Selling items for 1 gil is actually selling at a loss, It's just not good business.

Using Alligator pears as an example, as these are actually listed on Ragnarok as:

Alligator pear 1 gil each, in a stack of 99, totaling 99 gil.

When these sell, a 5% tax is applied and you do not not receive the full 99 gil. These items will sell to an NPC for 1 gil each, with no tax, so you will receive the full amount and immediately.

If another player buys them, gil is merely transfered from one player to another, whereas selling to an NPC will actually bring more gil into the game!

So my question is... Why?