It is possible to track the serials, if they wanted to; however, except for banks, I have not exchanged cash with a retailer who scans the bills at the point of transaction and if they did, they'd also have to tie a given ser no with me at the time of purchase. Since a particular bill will go through many hands, it'd be like trying to figure out who made a change on a system when it was done by 'root' or 'administrator' where the whole team logs in as root or admin. Basically attribution would be kinda inaccurate.