That's a standard question and one I've never understood. However, in your case the answer is simple: "It's such a great car and I enjoyed it so much I bought an R500." Don't refer to anything but the car for sale in the advert though, save it for the chat when they get in touch. That first contact is just as much about you determining if they're a time wasters as it is for them to find out more details about the car you're selling. Don't be afraid to be brief with them, if they're actually interested they'll want to arrange to come and view the car. Save any haggling for that point when they're stood in front of it and are actually offering to complete the transaction.