I was told that same lame story at one BB untill I pointed out the FAQ on the web page:
Question #12 The Limit is 3 per title per platform.
The FAQ can be found from the BB Home Page, at the bottom: PRODUCT SUPPORT/Trade in Center/Additional Resources/Frequently Asked Questions
Well I just got out of work and I wasnt thinking straight. Im just gonna leave it alone, ONLY this time. Guy was scared he was going to get fired if he let me trade in both the copies of MK that I had. I wanted to laugh at him, but it would of been an asshole thing to do.