What about account abstraction ? 

Let's say I built up a profile and want to give it to someone else... I give the new owner my nsec and he has no way of telling If I still have a copy of it or if it leaked before. 

Having nbups that are able to own keys to another npubs as nfts or something similar would be very useful in such case. 
Or are there other principles that can be applied?

How can this be solved?

#grownostr #asknostr #nostr #dev