I agree with a couple of the comments above. You don't need to justify why you've decided to sell your car. So personally, i would remove the second paragraph completely. It doesn't add anything to the advert.
You've said it's a reluctant sale - I would just add that whilst you have mixed emotions, it's time for a change!
Good luck with the sale - seems a shame that no sooner than you've finished the project, that you're selling, but I guess the problems with the rear-end have left a nasty taste in your mouth.
Best
Rush
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