If you don't manage to sell it somehow, you are left with big screen and no computer...
So you connect Mac mini to it?
Or else?
If you have a six year old iMac, which is about the age where it might become unusable, you are not actually left with a particularly large screen. And LCD displays do actually wear out, the brightness gets reduced, the quality of the display generally goes down, so it's not particularly worthwhile to continue using it.
If you don't manage to sell it somehow, you are left with big screen and no computer...
So you connect Mac mini to it?
Or else?