I think the reason he wants your account is to use it to scam people that are in this forum, he knows you are a reputable member in this forum and any information that comes from you will be believed by any forum member, he just wants to use your reputation to scam people.
Is good that you tagged him at least people will be very careful with him, I'm sure he may have messaged a lot of people too, this set of people should be banned from this forum in other to prevent this thing he did from Happening again.
Those that sell there account should know that they are putting people in danger especially if you are a reputable member in this forum, they will use your account to scam people which is not good so let's always kick against or reject such offer of selling our account.
If you are not interested in the forum anymore you can inform the moderators to ban your account or you can just leave it.