It wouldn't make any sense for you to sell it if that was the case. Why would you bother making an ad and dealing with customers if you had to give all the money away?
It all depends what kind of transfer it was.
If they gave it to you as a gift (Like it? Take it.) - it is yours now, you can do whatever you want with it and you don't owe anything to them.
If they lend you the thing (I don't need it now so you can borrow it but its still my) - you are supposed to return it when you no longer need/like it and if they agree to sell it you can sell it but you have to give them the money - they should pay you for the work related to selling it though, perhaps 20% of the earns or so.
If you are helping them getting rid of it (I have nothing to do with it, can you help me out?) it could be considered a gift but it would be a good taste to share the money earned if you are selling it, although it would also be good taste for the person you got it from to pay YOU for taking the bothersome thing from them but it wouldn't be good taste for them to order some of the money you earned or for you order them to give you some money. Social rules are that crazy.
From what you are saying your parents got it for free, then they lend it to you. So it is theirs. You either return it to them or ask them if you can sell it and take some money as a tip.