A friend help me in selling my car and refuse to give me my money, he has stopped picking my calls and i dont know what to do. i need advice on how to go about it pls.

Did you give him a letter of attorney to sell?
Have you informed the police?
Have you consulted a lawyer?

unfortunately, there was no paper showing i gave him a car to sell for me, am based abroad, and he knew am not coming home today. its really killing me, seeing me sweat going that way. The police is asking for 50k for mobilization. I have not contacted any lawyer since there is no document proving the deal between us.

You have to involve the police. that can not be avioded. take this step immediately.
Do you have copies of the car's documents? the police will ask for them.
Next time, you have to put your transactions in writing. Never,ever trust any Nigerian anywhere,no matter how close,even your blood sister.
Anything you do with a Nigerian must be documented.
Meanwhile,consult a lawyer for counsel. Dont write off this medium. there could be a legally workable solution.