I have an ATR 72-600 in excellent condition and just 3.7 yrs old. I am changing the structure of my company and I am selling the aircraft that I won't be needing. After I successfully transferred the flight plan to the new aircraft, and withdrew the pilots, I am still unable to offer the aircraft for sale.
You can sell it as soon as you've repaid a part of the loan.
The only condition is, that the starting price (for 90 %, 80 %, 70 %...) must be higher than the remaining loan for the aircraft - so the loan can be fully repaid once the aircraft is sold.
So if you're fine with offering the aircraft for 90 %, you'll need to repay only quite a small part of the loan.. it's usually doable. :)