it's allowed to create more than one account with the same IP , since it's forums here !!
Correct, it is not against forum rules to have more than one account. The question really is what do you intend to do with multiple accounts.
I also heard from my friend that they have many account especially joining free airdrops.
That is basically defrauding the airdrop and will lead to being given negative trust.
When you try to sell accounts, a moderator can give you a negative trust and the account become a worthless. If you involve in a signature campaign with multiple account too, the bounty managers will notice and you will work in vain.
It is not just moderators but any trusted member of the community (DT1 or DT2) that can give negative trust which will make the account wortless.
And what about selling account? Can person do it or it will also lead to neg trust?
Yes, all activity around account sales is likely to be for the purpose of facilitating fraud so it would be thought of as shady and appropriate trust would be given.