Follow selling is possible, but presumably counter-productive to both buyers and sellers:
- it only makes sense to buy follows from high-UA accounts, but when it does, that high-UA is worth more than selling it
- because other high-UA people frown upon selling follows, the high-UA seller will be unfollowed, losing more UA
- also interesting, because people can follow & unfollow at zero cost and any time, how should the follow buyer know if their money is well spent?
After paying, the buyer can be unfollowed, and the other way around, if a buyer only pays the seller after a certain time period, how does the seller know he gets paid at all? Of course "smart contracts" could be a way to make sure the buyer pays and the seller follows, but that would be public and therefore risky to lose followers.
So "thinking practical" makes sense a lot in order to guess what could happen.
PS: did you already take the time needed to read up about, and understand UA, as you wrote above?