Adds a slot to the market for the entire game, so you go from 6 cards in the market to 7; there was a similar upgrade in Crossfire. What post did you read that changed your mind?
These are also additive. ie 2 Half-Elf characters in the party will make the market size start at 8, and so on.
Additionally, the market size can be modified mid game by a few special circumstances (Ryana Amcathra and Gnoll Witherling Encounters, King's Fist Dragonfire card, possibly more)