Deals on eBay and other secondary markets

I am the maintainer of Brickonomics, a platform to compare LEGO prices, and I recently added the feature to check prices on eBay and Brick Owl. For older sets the deals via these platforms are much better than from regular retailers. For example, the UCS Cantina $550 on Amazon and Walmart but it can be found for over a $100 cheaper on eBay: see https://brickonomics.net/set/75290-1 under the "Secondary Market" tab. Is this a good addition for LEGO investors? Or do you only purchase sets that...