I bought volume one of Spice and Wolf today, and I read the summary on the back. I understand that the main female character is a wolf and the main male character is a merchant, but he doesn't just sell spices, right? Is the name just meant to sound badass, or was I wrong and he really only sells spices? I'm just not getting it.
This is cannot be explained without spoilers.
The phrase "Spice and Wolf" was used by a merchant to describe the two main characters later on in this series.
It was also repeated by the author in the afterward of volume 16 on page 299:
...But as the world of Spice and Wolf is not about viewing the world in itself, but the "Spice" and the "Wolf" (Holo and Lawrence) in it , it's actually the antithesis of that universal metaphor , so in other words... et cetera, et cetera, et cetera, ad nauseum.
According to the afterward of volume 16, on page 297,
The title Spice and Wolf is a twist on the French economist Jean Favier's Gold and Spices: The Rise of Commerce in the Middle Ages(translated by Hidemi Uchida). Thinking back to when I read it, I recall thinking I'd love to use things from this, which gave me inspiration for the first volume.
Spices, along with precious metals and silk, are commodities associated with trade (have you ever heard "spice trade" or "silk road"?). Since this is a series heavily centered around medieval commerce, the "spice" in the title is meant to evoke that the series is about trading. As mentioned in the quote, the author was specifically inspired by Jean Favier's book.
The spice trade was quite important and lucrative in medieval times. Here's Lawrence's thoughts on the matter from volume 2, page 24:
Lawrence had bought the pepper for a thousand trenni, so that meant a profit of 560 pieces. The spice trade was indeed delicious. Of course, gold and jewels -the raw materials for luxury goods- could fetch two or three times their initial purchase price, so this was a meager gain in comparison, but for a traveling merchant who spent his days crossing the plains, it was profit enough. Some merchants would haul the lowest-quality oats on their very backs, destroying themselves as they crossed mountains, only to turn a 10 percent profit when they sold in the town.
Indeed, compared with that, clearing more than five hundred silver pieces by moving a single light bag of pepper was almost too savory to believe.
Further reading on Wikipedia: Spice Trade