Buying a lien to flip, can it work?

[Samson Taylor]

I've found a nice 4/2 MH, owner asking 21k, which is her lien on the home needing to be paid off.

Please tell me if this scenario is doable.

***Sally owes 21k on mortgage

***I with little to no money sign paperwork accepting responsibility of the 21k mortgage, which will be paid off paying the same monthly payments as Sally

***I then find a tenant buyer and process a new mortgage with them at 31k × 11.2% interest.

***now my tenant is paying off the intial 21k mortgage, whilst fulfilling their 31k × 11.2%. Mortgage to me. 

***I then sell my 31k note either partially or fully to a private note investor.

Can any of this be done? 

Will it be a problem that Sally's mortgage isn't paid in full till I have a note investor pay the 21k off?

Basically can you accept mortgage responsibility, then create a mortgage on property that's under a mortgage already. Only to sell the new mortgage note to a note investor?