Hmm, good question. Tbh I'd write an ad like this:
Nissan 350Z GT Coupe, 2003
55,000 miles
FNSH
6 months tax & MOT
Then some blurb about how long you've owned it, how lovingly you've looked after it, no expense spared, only used at weekends, that sort of rubbish highlight any bad points next, be honest but don't make them sound the end of the world, then finish with a few lines saying just what a great example it is and you'll miss it.
Generic rubbish perhaps, but it works and will make your ad stand out. Front load it with the important info so you don't waste people's time, and then gush but don't put stuff in like electric windows, airbags etc as no-one cares and it's irritating to read.
All just my opinion and I should state that I've never sold a car privately, so this is only what I like to read when browsing ads!