But it was me, the shopkeeper

[My Self]

You know how in certain other RPG games, you encounter merchants inexplicably deep in the dungeon?

I was thinking of inserting one into a campaign. The merchant would appear in the dungeon every few floors, offering to buy/sell potions/weapons/other junk at a reasonable price. Then, once the players reach the bottom floor/boss layer, they find a room that is almost empty... except for the shopkeeper.

What are some puns that a murderous shopkeeper could use in battle? I was thinking: "Sorry, we're all out of potions, but everything else is half off! Which half do you want off?" when the merchant makes a full attack with a sword or axe

[Bane Wraith]

(Upon being attacked) "Oh, that one's going to cost you an arm and a leg..."
(Upon casting an offensive AoE spell) "Clearance sale! Everything must go!"
(Upon making an overhead strike) "Customer Satisfaction Guillotined!"
(Upon dodging a swing) "Hey! No refunds!"
(To the halfling) "...But you're already half off!"

[Klorox]

(if casting mass reduce person) "Short on funds, are you?"

[Tacticslion]

D-Daggummit. XD

[Rub-Eta]

Have him sell kebab, explain the Bargaining rules from Ultimate Campaign for five minutes, have the players use the Bargaining rules to buy a kebab (but only this one time, never mention them or use them again after this). It's not until after at least one player has bought a kebab (to what the player thinks is a reasonable price but is actually outrageously high) that the shopkeeper reveals his true intentions and pulls off his disguise.
If you do this, you'll be my hero.

[GM Evil]

This brings that dwarf and his "son" from Dragonage Origins to mind.

"Enchantment? ENCHANTMENT!!" (As he's surrounded by body parts, gore, and broken furniture."

[QLMMaster]

Shopkeeper: *full attacks with two weapon fighting*
buy one, get one free!

[Zombie Boots]

When the PCs enter the room in the first place there should be a bell chime.

(2nd Attack) Didn't like that first one? Let me Slash my prices
Here is a deal to die for!
(Attempt to Escape) Time to recoup my loses
(AoE) Bundle Deal
(Fire) This item is red hot!
(Thrown Attack) Lemme show ya my sales pitch!
Try out this new product! It's Cutting Edge! (Player's may loot item: Cutting Edge)

In truth OP, the way I would play such a character, there would be little you could plan dialog wise. Crave out a overbearing salesman personality and play off your PCs. An over the top voice might help if your group is into that.
During combat if they miss with a sword, have him offer them a better one for a price. Heck even if they HIT with a sword, have him offer one that is better than "That old thing"! If a spell fails have him offer the caster some NoMussNoFuss-Spell-Polish "Guaranteed to help with Saves, Power, Cleans your familiar, Promises a good night sleep, Smells nice, will clear up that rash, sever nausea side effect in 80% of users, and did he mention it comes with a free razor?"

[Kobold Catgirl]

Have him at some point get an extra attack (such as through Haste). Then, at the end of a particularly nasty volley of attacks, issue the last one with:
"BUT WAIT—THERE'S MORE!"

[Magus Black]

"Over here stranger, got somethin` that might interest ya` heh heh heh"

*Cues Boss Battle music*

[M1k31]

It might also be interesting to describe the rooms layout like a store with each shelf containing some item of interest(claiming an outrageous sale) only truly discernable while standing nearby... while also being a trap/hazard/haunt... it would be cool to have the players scramble to figure out how to loot the room rather than all piling on the guy 1 vs 5 or 4.

[UnArcaneElection]

If you actually try to buy something peacefully from the shopkeeper, but you're in a hurry, this happens.

[Dr Styx]

love the store idea M1k31, but make it a China Shop with Bull's as the Shopkeepers pets.

[M1k31]

Dr Styx wrote:

love the store idea M1k31, but make it a China Shop with Bull's as the Shopkeepers pets.

Druid cat followers with bull familiars with the feat that allows them to squeeze?