If my Friend left her belonging in your house for 2 weeks without mentioning anything,and I try to sell it for profit,is it illegal?

If he finds out and says this sentence"I want you to give me back my belonging anddollars cash for compensation and I will forgive and forget" is this blackmailing? He left her item in my house for1 week before mentioning it is lost to me.now he found out I tried to sell it online what can he do to me? Should I follow his instructions (he treats me badly that's why I did that)