If I left my rather pricey bicycle at a friend's house for about 4 months and he sold it without my permission, what can I do?
He made no effort to contact me to take any action to move or retrieve the bike. He knew that I was going to leave it at his house for an indefinite amount of time.
Your friend knew you had not abandoned the bicycle. You could sue your friend for conversion for selling the bicycle. Conversion is the unauthorized assumption and exercise of the right of ownership over the pesonal property of another to the exclusion of the owner's rights. Conversion is an unauthorized act which deprives an owner of his property permanently which occurred when your friend sold your bicycle.
Your damages (the amount of compensation you are seeking in your lawsuit) would be the value of the bicycle. In a lawsuit for conversion, your damages would be referred to as a forced sale which would be the value of the bicycle. Depending on the amount of your damages, you might be able to file your lawsuit in Small Claims Court. Your damages should also include court costs which would be the court filing fee and process server fee.