"I paid $120 for my Dreamcast, and I was reading last night on the internet that it sells new for $99."

"Wait, wait.  Not keeping up here.  On The Internet?  Never heard of it."

"Sure...  So you see, I paid too much for my Dreamcast and I want some money back."

"A noble goal.  Well, good luck on your quest." 

"I bought my Dreamcast from you."

"Does that make me the protagonist or antagonist in this story?"

"What?"

"Am I the good guy or the bad guy?" 

"Uhm, you see I paid too much for my Dreamcast."

"Yes, yes.  Further you also can read some book called 'On the Internet' which sounds quite interesting."

"Just give me some money back off the Dreamcast I bought from you."

"No."

"No?"

"No."

"You have to!"

"Explain why."

"You charged me too much."

"First, assuming that was even true, why would such an act invalidate our previous transaction?  I don't recall me forcing you to buy the unit, plus I recall you being quite excited to be receiving the unit."

"So?"

"Continuing on, in this Internet book you were reading, perhaps you didn't take notice that the price of the Dreamcast is $99 US.  Now both the United States and Canada both use dollars as currency, though they are of different values.  Why, if the Dreamcast was $99 in Taiwanese dollars, I could buy one for less than what I spent for two movie tickets last night."

"uh"

"As you may or may not have concluded by now, the price you read of $99 for the aforementioned Dreamcast would be for 99 American dollars, which translates to about 153 Canadian dollars.  Being that the Dreamcast I sold you yesterday was for $120, I conclude I sold it for too little though it is too late to amend the transaction.  Or is it?  dot dot dot..."

"I'm sorry.  I didn't realize the prices were in American."

"Spoken like a gentlemen."