Is it legal for me to sell my evicted roommate's junk car after 30 days to get money he owes for rent?

Not if you don't have title to it, even if it's junk. You would have to file suit against him, get a judgment in your favor and attach the judgment to the car. If it's low value you should be able to do so in small claims court and your case should be cut-and-dry. You're also luck you have an asset just sitting there - most people have a very difficult time locating assets even after a favorable judgment. Good luck.