If supply exceeds demand, and you're selling high, the undercutters will sell first, and will justifiably set the new price. Learn to read the market and adjust; if there's too much of a product, the price is obviously going to go down. What you THINK it's worth, or what you WANT it to be worth is totally irrelevant. You can TRY to buy out the low-cost stock and sell high, but this will only work for as long as you're actively patrolling the marketboards. Which can, indeed, result in decent profits, if that's your idea of fun.

If demand exceeds supply, and you're selling high, the undercutters will sell out fast AND your stuff will sell, no matter what price you set it at. In fact, you might want to bump your price up a bit. Undercutters can TRY to bend the prices, but they will fail - they simply don't have enough product. If they succeed, then this is not a demand-exceeds-supply case (they DO have enough product, so there's no shortage of supply), so see above.

Complaining about undercutters is always foolish. Either they are reading the market better than you are, meaning you should adjust to match them, or you're reading the market better than they are, in which case their actions make no lasting difference at all.