I recently took a trip to Busan with my sister, and the fish market was my absolute favorite part of the excursion. Here's roughly how the fish market thing works:
When you walk into the massive building, you'll see scores of vendors trying to slang their catch to you for cheap. They're not aggressive or pestering; some of them can speak-a-da English fairly well and are open to haggling. They'll be delighted to see your foreign face. (Continued...)
My sister and I picked out octopus, flounder, abalone, two massive mussel-looking things, and two creatures that looked like red hand grenades. The vendor threw in a free sea penis (I think that's the name) as "ssabisu." A lady appeared out of nowhere to take our food upstairs where there were dozens of little restaurants filled with bustling ajumma who cook up your pickings right on the spot.
It's the epitome of freshness and the prices were reasonable--be sure to check it out if you visit Busan.