As my character levelled up, I saw that the number of slots seen offered (BEFORE I place any trade) increased progressively (now showing 0/4).
However, as soon as I placed a single trade, the interface requires me to pay to 'create an additional slot'; this even though I tried it once.
This effectively makes slots 2/4 to 4/4 non-existent, and here is the bug.
As for paying for USING existing slots, that is simply not acceptable until I can actually produce coins, which is still several levels away.
Not to mention that the AMOUNT required is at the same level as the value of the only thing I can currently sell for coins to try to balance my resources, making any potential paying slot irrelevant to the purpose.