ONe thing--make sure you make copies of any emails, correspondence, and transaction papers, both to the right person and the wrong one--I ended up faxing 11 pages of stuff to verify the right and wrong of my situation---but mine wasn't a mistake it was fraud, and I had to fill out a police report with my local dept. before they would take me seriously, and that was a royal waste of time, considering seller was on the West Coast and I am in the midwest.
I would think they should be able to resolve a unintentional mistake without making you too crazy.
Keep everything either real copied in a file or placed in save somewhere you can get it.
I have found paypal to be handy---however it is more seller/acceptor friendly than buyer/sender friendly if there are problems.