You, my friend, need to wake up and smell the roses.
Just have a look on the trader, and you will see that for the 10k this guy is asking you can get cars that are/have:
better performance
better aesthetics
better gas mileage
better mileage
no rust, 0
etc.
You need to realize that the 240 is NOT a fucking Ferrari, and they will not hold their value because they are un-reliable rust-buckets. Cars are cheap these days, and in order to compete with that guy selling his 2002 Saab 95Aero for the same price as your 89 240, you will have to lower your price significantly. Understand?
For example, I will soon be selling my 97 Subaru Legacy LSI for $4-5000, the same price that sorry 240 should be. And even though the Subaru is auto, I am sure I can out-handle that 240 on a twisty road with my stock car.
But what do I know, I couldn’t sell my car for 10k…