I think it is fine to post up for sale ads and discuss one's personal view on an offer in the public domain - however, that shouldn't imply, accuse or humiliate any seller who is fully entitled to place their valuation on their car, especially if the commentator hasn't viewed it. Nor do I think that any ensuing communal discussions should infer or imply a notion of 'market value' applied to each unique sale, for fear of hinderance or embarassment to the future owners.
Of course we here are all interested in a 'market value' - but I'm sure we all realise that such a concept barely exists in the real world, especially when one factors in passion, scarcity and such varying levels of condition in 20 year old cars!
How might a rational discussion go in this case:
http://www.carandclassic.co.uk/car/C163667/