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.
RE: DIY Income From Thrift Store Flips -- $65 Spent Turned Into $1,443 Sales Revenue (22x Return) -- Buy Low, Sell High!