I am indeed a Lego fan, and also sell a bit online. It's pretty rare to find any Lego in thrift stores and the like around here, especially still sealed.
Over the years I've had my share of success from thrift shop flips, though not to your level by the sound of it.
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.