@Dogma, you are quite right in suggesting that forums will always have a proportion of lurkers. That said, an online forum can only thrive with the contributions of its members of their thoughts and experiences.
As this forum is for a car which went out of production a decade ago, I feel its best days are behind us and that it needs more than ever for its dwindling membership to actively contribute for it to survive. Quality of the member rather than the length etc
If you think it is harsh to delete posts of this nature, then perhaps it is reasonable to ask these people to make a donation to forum running costs for hosting their advert?