A lot of accounts have higher trust rating now, how many different positive trust feedbacks does it have? How long ago can you sign a message from associated with this account as you have not stated and that is a big deal. I have to give you a heads up for a newly Hero account you are not getting nowhere near 3btc! Sorry to break it to you, you will be very very lucky to get 1btc 
Thank you for your answer
Honestly I'm not very skilled in cryptography, how can I sign a message and why it's so important?
No problem, the reason it is 'so important' to have a signed message posted by the account a long time ago is to prove that the account is owned by you and that you transfer it to the buyer upon purchase. Have you posted a btc address from the account in your 1 and a half years that you still control the address? Here is a brilliant walk threw how to sign a message https://bitcointalk.org/index.php?topic=990345.0 . A signed message on a hero account sale is a must. PS 1year.6months here and you don't know how to sign a message or you don't know much about crypto, give you benefit of the doubt but that does not sound right to me.