Yeah I have the full set, plus all the Chronicles and Megamorphs, and the two alternamorphs books, and a meet the characters book from the TV series. They are all in GOOD condition, save for a few books from the beginning of the series that have a tear or two in the front cover(shame).
I would NEVER EVER EVER sell them, in fact I'd like my children to read them as they grow if they would like to. I'm currently re-reading the entire series including the extra books, which I don't think of as "extra", in chronological order. When I'm finished(on #21), I am going to get a bunch of those sealable air tight baggies and vacuum seal all of them and then they go into storage. I want them to remain in decent condition.
But hey, by all means, if you'd like to sell, go ahead and commit yourself to a life of regret. Hehe...