designed with pick up raids in mind

Raid bosses aren’t designed with pick up raids in mind, but the bosses should get more challenging as a raid progresses through the instance. Naxxramas is perhaps not a good benchmark to set raiding expectations against for a couple of reasons. It was content that a number of players were already familiar with, and it was also tuned to a difficulty that was or world of Warcraft a bit lower than we would have preferred.

I too vividly recall the start of Burning Crusade, and pick up raids were anything but common at the start of that expansion. In fact, the idea of a pick up raid was flatly laughable for quite a while. They didn't really appear with any regularity until the mid-portions of the Burning Crusade expansion, and only after most of the participating players knew the fights very well, or just overgeared them by a pretty wide margin. Even in Wrath of the Lich King, pick up raids didn't start becoming super common until a tier or two of content had already been released. The scenario you mentioned in your original post, of constant pick up raid spam in trade, didn't really occur until a bit later.