First, I'd just like to point out that THEY SHOULD be undercutting you. If supply increases without an equal or greater increase in the demand, the price should fall.

Undercutting will always and should always be apart of any market.

It's rather complicated to explain in full detail, but the basic idea is simple. The items are obviously so easy to get, that the supply is constantly being increased far above the demand. Or, someone is spending their time lowering their prices constantly just to ensure their goods sell first. They lose gil, and the supply never changes so you should be able to sell still. But considering how you state that you haven't sold in weeks, then I'm assuming the supply is overly abundant.

Which means simply, the market does not really need anymore black sole, and it's trying to encourage people to move into more productive things. Or if you just want to keep selling black sole, then you will have to keep selling for a cheaper and cheaper price.

Really it's all perfectly normal.