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.
References In-story to the Title
The other answer to this question uses references of the title in the story. These deserve a place in this discussion, however, these references do not actually explain where the title comes from, as I'll explain. I believe the character the other answer refers to is Marheit. However, in the English translation of Spice and Wolf, I could not find an instance of him actually calling Lawrence and Holo Spice and Wolf. Instead, it is the narration (Marheit certainly prompted the reference though, and he probably says it in other media, like the anime). Here is the quote, at the very end of the first volume:
It seemed as though the travels of this pair would last a bit longer.
That is to say, the travels of the wolf and the spice.
While this reference is interesting, it does not explain where the title comes from. This is because it is in-story content. Unless the author wrote the whole book without a title, then looked at his own story and thought that this point at the end was important enough to make it the title, it cannot be the reason the story is called Spice and Wolf. And actually, when the play that Marheit is referencing is examined, it supports my initial claim that spices are linked to trade. The play is a morality play about a merchant trying to convince a demon to eat someone besides him. Here is Marheit's recounting of the end of the play and Lawrence's reaction (Page 227 of Volume 1):
"'The most succulent human is before your eyes- he has carried spices day in and day out in his quest for money, and his fattened soul is perfectly seasoned,'" continued Marheit cheerfully, gesturing expansively as he related the tale. [...]
"It's a religious play that the church uses to preach moderation in commerce," he explained. "[...] Pepper is surely appropriate for a merchant about to make his fortune, I think."
Lawrence couldn't help smiling at the amusing tale and Marheit's praise. "I hope I soon have a body suffused with spices myself!" he said.
Rather than an explanation for the title, this merely highlights the author's understanding of the connection between spices and trade.
Further reading on Wikipedia: Spice Trade