Right now you can sell your deposit via vTokens, but a well developed financial requires also the possibility to sell debt via special tokens, in order to facilitate accounts with big debts to get rid of debt and allow others with solvency to buy it with a discount on market (the profit for the buyer) and provide more liquidity to the system.
Notice this is equivalent to liquidation, but initiated by the own borrower in order to avoid heavy looses by the loan interest rates. If he waits for forced liquidation, he has to wait for bigger accumulated debt.
This will allow the borrower to put part or all his debt available for immediate liquidation for avoiding worse losses , and people eager to liquidate, to start to do that without need to wait for target account to reach health zero.
Update: this proposal has been superseded by this other one:
which is more general and specific, and has the same purpose (easing liquidity pressure)