My first question is what are the restrictions to selling an accounts???
There aren't any official restrictions from the admins or moderators of the forum as far as I know but when someone tries to sell accounts then reputed members of the forums leave tags on the profiles of those users because they really don't like the activity of account selling or buying. It's always recommended to avoid buying or selling of accounts on this forum and that's why it's disliked by the community of Bitcointalk to sell accounts.
Does this mean that if I have always had two accounts and for any mistake whatsoever like plagiarism. And one of my account is banned, am I not allowed to own any other active accounts??? Let's not forget that it is not illegal or against the rules to own multiple accounts. Are these rules contradicting themselves or are there other hidden criteria to such effects???
It's legal to own as many accounts as you can but keep in mind that if any of your accounts get banned due to any reason then all of your other accounts are also going to get banned if someone finds connection of those accounts with your banned account. You can literally have as many accounts as you prefer to have and if you follow the rules of the forum then you won't face any issues even have multiple accounts. But, it's not allowed to participate in signature campaigns with your alt-accounts and if a manger founds that then you may get tagged for that activity.
Does this mean all other of my old accounts remains relevant and active despite my initial ban on my other accounts???
When one of your accounts is banned on forum then your other accounts won't get affected from that particular account's ban but remember that when your account is banned and you continue to posts from your other accounts in sense that you really don't care about the ban, and when someone finds out that your current unbanned account has connection with your banned account then surely your current active account will also get reported and might get banned.
It's always recommended to create new accounts only to create a ban appeal threads on Meta board about your ban and when that appeal is accepted then you should continue using your other accounts or even that banned account which gets unbanned if your appeal is accepted. That's the safest way to use the forum and trust me if you don't break the forum rules then you won't get banned in the first place.