Translation:I want to sell my car.
Exactly! In the early part of this course they made it clear that 想 meant "would like". In other non-Duolingo courses I have studied as well, they also make it clear that 想 is "would like" and less definite compared to 要. But here with utter inconsistency they mark you wrong if you write "I would like to sell my car." And of course, nothing has been done since you wrote this 4 month ago. I have also reported it to no avail as well.
"Would like" is exactly the context you would use 想 here because if you want to sell your car you cannot be certain it will happen.
This is half a course and the maintenance on the answers is appalling.
Duolingo should really remain consistent with their use of 想 and 要. 想: would like 要: want
When you say you "would like" to do something then use 想. The distinction is that 想 presents a less demanding tone as 要. Although both are fine, Duolingo should accept 想 as "would like / want", not just "want". All that we ask for is consistency.