Welcome to the club. They're not super common, but some stores will have them and price them like general toys without any research. If they're open, I usually don't buy them unless they're an expensive kit I can part out. For example, I recently bought Mindstorm #8527 for $10 and confirmed all parts are there except 4 tiny pieces. I also just found this one sealed on Thursday:
My best one was a large sealed Harry Potter set for $10 that sold for about $530.