I will say that a buyer has assumed risk when an item is being sold "as is", however if the seller is gauged to be familiar with the car (as he was in this case), he should have been more than willing to disclose the little things that would surely vex the owner at a later date. It is fair to say that it would only be a matter of time until you found out about the sideskirt.
Both parties are at fault, but the seller, being a prominent paint shop owner in a very competative industry, should have done something a little more to rectify the situation.
As a result of this thread, one or more members will decide not to take their car to his shop, and that is really what it comes down to. Opportunity cost is a bitch. I'd rather put out a small fire than deal with damage control.