Allowed but not encouraged because most of the time, accounts that are being sold are hacked accounts. As for your question in regards to activity, you need to have a post for every two-week periods in order to get the potential 14 activity points on that round, and 14 posts on that week if you want to have the activity points reflected on your account.
In order to get 100+ points, you need 3 months and 14 days.
Yes you can sell it and buy but as much as possible dont buy especially you dont know well the person maybe to those who are knownby you. And you can sell but if you
have benefits now and it is quite good dont sell it already.