I bought my car in the Summer, I got my fireball 2 weeks after i got it so my stock catback exhaust is basically brand new, i kept it in my storage the whole time.
It was up for sale a while ago and this guy picked it up today, just a couple mins ago he gave me a call and told me that the thing is rattling like crazy, he checked the bolts and stuff and there are no air leakage so it can't be the bolts not tightened enough i guess... what else could it be?

 

maybe the headshields, loose bumper, or maybe even the license plate. I know that my license plate rattles at idle..

 

check this. there is a O ring that is between the car and the down pipe i forgot to put this back in after i installed a new cat. my rattles at driving speeds. 1k - 3k. partial throttle. hope this helps

 

Did he check the hangers? Make sure both hangers are on correctly or the metal pegs will bang against the bottom of the trunk.

If it really was the exhaust that's rattling, he would have noticed it when he purchased it or did he not check. You should aways shake the exhaust canister to check for "loose" insulation when buying a used exhaust.