sale of collection accounts

i have a credit card that was charged off several years ago and each time a new collection agencie buys it they charge thier fee and intrest. so now i went from having a $5500.00 bill to a little over $17,000.00. anyway my question is i know after seven years a bad debt should be removed but since these companies keep selling the account does that mean it extends the seven years each time or does it still go off the date of the original charge off? thanks. also i live in FL, not sure if that matters or not.