These are tough times for the used bike market! There is near zero profit in bike restoring right now, usually they sell for less then then cost.
This is just what Im seeing in the marketplace right now, not in particular to your bike. A good time to buy, not so good to sell.
The more desirable vintage kx's are the pre 79 models. Try to hold onto your bike to help regain some value.
Im looking at an 89 ktm mx250 in decent shape, but needs motor work, but Im afraid to buy it even though its a rather rare bike