The gemara tells of a strange discussion that took place between two Egyptian sorcerers and Moshe Rabbeinu. They challenged Moshe that his "tricks" of turning the stick into a snake and water into blood are unimpressive - תבן אתה מכניס לעפריים - you are bringing more straw into a place abundant with straw. Egypt had plenty of magic so that Moshe's magic didn't impress them. Moshe responds למתא ירקא, ירקא שקול, to a place where vegetables are abundant that is where you bring your vegetables to sell. Rashi explains that the entire dialogue was silly and pointless. Since they challenged Moshe with a silly parable, he responded in kind. The message in Moshe's statement seems to be that where magic is popular, it is appreciated. The Maharsha explains that they specifically chose a parable of straw which is all the same. They were indicating to Moshe that his magic was no better than theirs. To that Moshe responds with a parable of vegetables which are all different grades and qualities, indicating that the signs he was preforming were well beyond their capabilities.
I would like to suggest an alternate approach to the conversation. The concept of bringing something unnecessary, such as coals to new castle (known as a major coal exporter), is foolish only if the goal is to sell your coal. However, if the goal is not to peddle your own wears but to show contrast in order to undermine what is already there, it is very logical to bring a higher form of the same. Their argument to Moshe is that he won't be able to use his magic to convince the Jews or Pharoh that he is an agent of G-d because it is like bringing straw to עפריים - a city that thrives on straw. To that Moshe responds that he doesn't need miracles to convince the Jews of Hashem. The Jews inherently will believe that he was sent by Hashem to take them out as promised, but they are being fooled by the magic of the Egyptians. Therefore, Moshe responds that he is bringing his high grade vegetables into a place that has low grade vegetables (as the maharsha explained), with the goal of undermining their magic. The magic of the Egyptians was the curtain blocking their vision from seeing Hashem. Once Moshe is able to show that their magic isn't special, he removes the curtain and enables them to clearly see him as the shaliach of Hashem.