Well if that's what they want why not? Ask questions and use the answers.
So [wizard], where is it rumoured that you can buy the spells of your dreams and magic staves of power? Oh really? Where's that on the map? So [thief] you've been there right? What's the word on the street about where this magical miscellany comes from? Is it plundered? Stolen? Crafted?
You've heard of the civil unrest in this city [paladin] What's the problem that needs purging in this arcane den of iniquity?
[fighter], what's the one fabled beastie you've always wanted as a trophy? What magical weapon is rumoured to be its downfall? Who has it? Where is it hidden? Hmmmm, sounds like you are spouting lore to me.
That sort of thing. Give the players (rolled) authorship over the sorts of magical things they want to buy (and why). Dig a little and generate histories and fables and WANT for these items. Ascertain where they need to adventure to buy them, who they need to buy them from and establish their fictional power in your Dungeon World. Finally, detail in your own fronts the antagonists who want them for their own nefarious ends.
I cobbled these 'Town Moves' long ago, inspired from AW and Apoc D&D moves during the first iteration of DW, and use them still. They give you nice story hooks along with the purchase :)
http://apocalypse-world.com/forums/index.php?topic=1183.0
For more expensive goods and services, you should expect to make particular arrangements through the following moves. You can’t just wander around the town square, with trinkets and treasure a jingle-jangle and expect to find your heart’s desire.
Purchase Specialty Goods and Services (charisma)
When you go into a bustling market to pay money for goods and services through legitimate channels and its not obvious whether you should be able to just go buy it like that:
Approach the appropriate NPC and choose one or more things you want to acquire from them:
•Services
•Succour, protection, or lodging
•Opportunities for adventure, fame, or personal gain (see adventure hooks)
•Specialty, magic, or rare stuff
•Information on a specific topic
Roll + CHA. On a 10+ Choose 2. On a 7-9 Choose 1
•It’s not expensive -1 Treasure
•It’s ready right away
•No strings attached
If you fail, you can have it, but it will be expensive and you’ll need to undertake an adventure first (see adventure hooks).
Go Through other Channels (varies)
Use this move to get stuff that’s not available through regular channels, or to use less legitimate means to get what you want. Each channel uses a particular attribute and has its own risk (in parenthesis after the channel name). A given town might have more or fewer channels, including channels not listed here (which the DM should make up).
•Get what you want through threats, violence, or intimidation (strength/embattled)
•Get what you want through fraud, theft, or trickery (dexterity/discovered)
•Deal with the local temple, church, or cult (wisdom/entangled)
•Engage in risky arcane research (intelligence/ensorcelled or magical mishap)
•Deal with the local criminal underground
(charisma/double-crossed)
Roll 10+ then Choose 2:
•There are no strings attached
•You don’t promise anything in return
•It’s not a matter of common knowledge that you got it.
Roll 7-9 then Choose 1:
•You can have it, but you’ll have to undertake an adventure first (see adventure hooks)
•You get it, but the risk comes true.
On a failure you don’t get it, AND the risk comes true.