The biggest mistake as you hint to above Joely is that the value of cars is not what they are advertised at. No one really knows the true value i.e. what someone would buy one for as sold prices are not advertised but I suspect they are around 10% lower than the advertised prices on average.
But it is the way of things as sports cars age, they become rarer, good condition ones hold a premium (in most cases, there are maligned sports cars that will never appreciate) but its not unexpected that Z32s will go up in price. I sold my 02 plate VX220 for £7750 two years ago, I would be looking at paying around £8500-£9000 to buy it back now.