Oldest scam in the book.  Common sense says no one from TX is buying a $100 treadmill in CT even if it was free and definitely not dropping $1700 to move it.  Sorry to say it but if you can't figure that out I don't know what to tell you.  And no one sends more money than your item is worth legitimately.

For those living under a rock and don't get it the victim laundered the bad check through her good account and deposits a legitimate check into the scammers account.  It has been around as long as the internet.  Best variation I have heard is someone pays the legitimate amount then calls a few days later claiming family member died or some other emergency and they need the money back.  Only way you should ever deposit a check of any kind from an unknown person is at their bank branch and take the cash to your bank.