Having defended many a lawsuit, I'll add my 2 cents.
First, I have a tendency to agree that when slapping an engine on a bicycle, whether done by an eBay seller or not, a guy has to do his research and take responsibility. A very little bit of research would have revealed this site and others where the fender issue has been common knowledge for years.
Second, and this is a little more inside baseball. One of a couple of things happened with this defense- either the defense attorneys did not get the bracket (or exemplars) on purpose (because they had a pretty good idea of what they would find), or they did, found that the results would hurt them and chose not to use the expert who did the tests. It is common to have a metallurgist do testing after metal failure/fatigue incidents, so that there is no mention of a metallurgist for the defendant, tells me something.
Third, even with a vibration-prone 2 stroke, one week of use doesn't seem like much to destroy the bracket...something seems odd here.
Fourth, this is why I do not have fenders in the traditional sense.
Fifth, it seems odd to me that the defendants would not Google this issue. If they had, they would have been clued into an additional defense that the end user should have taken preventative measures.