oh, there's SO many ways to get back at those types of players the OP mentions that it's made playing a Cleric more fun then I ever thought possible. My first bit of advice is learning good comebacks:
PUG'r: "Help, I have stat damage!"
Cleric: I'll sell you a Lesser Restoration for 50gp, or a potion for 10k!
PUG'r: "I have 1 negative level!"
Cleric: Sweet, now you're the approriate level for this quest!
PUG'r: "Gah! I just got cursed and despaired by a Mummy! Where the hell did that come from!?!?"
Cleric: ...
The Party: "LOL"
PUG'r: "ZERG ZERG ZERG!!!"
General: *DING*
PUG'r: "how come you didn't heal me!?!"
Cleric: You were blocked by the, uhm, yeah...you were blocked.
Squishy: "HEAL ME!"
Squishy: "HEAL ME!"
Squishy: "HEAL ME!"
Squishy: "HEAL ME!"
General: *DING*
Squishy: "what the hell!?!?"
Cleric: It's cheaper to resurrect you.
The next best thing to a Haggle item is a Bard and they rarely ask for repayment when they purchase the 100 Heal Scrolls you requested.