Are you legally allowed to sell off a car to recover money owed??

Basically car has been sat in workshop for nearly 6 months now.
Customer hasn't turned up about it nor the friend who got me the car / job!!
Car owes me some money for parts plus the fact that it has been a major obstruction in the shop as it needs an engine and is a pusher!
I have threatened to sell it but no one seems to be bothered and it is now pissing me off!!
Tempted to leave it outside but dont want it broken into etc!!!

Car in question is an E46 320d Compact in mint condition exterior and interior with 18" BMW alloys and cream leathers.
Any ideas what they are worth?