In essence, you need to start with a car which is in current demand. Then you need to make changes which are congruent with current trends. Cost invested is paramount to a positive return. The right buyer is key top dollar. Keep watching those auctions. They are an excellent indicator of the economy. You'll also notice the auction price only goes sky high when multiple bidders are present. Your best selling option is always to have several buyers lined up before you even start. Selling privately avoids auction fees. No good to take it to auction for an extra 20% if auction fees, transport costs, lodging, etc cost you 18%.

Research. Keep costs down. Sell privately.