I lived in California for over 40 years and have seen lots of those warnings on multiple items. However, I've also seen the same warning in diners, beauty shops, appliance repair shops, gardening centers, etc. I think it's all a matter of exposure, weeks or months vs. hours or minutes. But, there is no concern on the part of the people who work in all those places and there's no news about all those people getting cancer and suing the shops or the government, so I wouldn't worry about it. Don't take the oven apart, don't mess with the gasket or innards.

I think you have to do what YOU feel most comfortable with. If using the oven is going to make you nervous or make you worry, you should just return it. If you can use it without concern, then keep it. Alondra is right that there are lots of hazards out there if we look but we can't do things that cause us worry.