There are only three ways to unlock the GPS.
1) Get the PIN from whoever you bought it from.
2) Take the GPS to the location it was at when the PIN was set.
3) Send the unit to Garmin to have it reset. For return information: CLICK HERE
If you can't get the PIN from whoever sold it to you online, it is probably a stolen unit. You should contact your local police dept with a the serial number to see if it turns up in their database. If it is stolen, you may be eligible for a refund from the online auction house because you were defrauded. At the very least you should file a complaint with the auction house to prevent this sub-human from screwing other people the way he screwed you.