In a perfect world, with so-called 'perfect knowledge', yes. But fleabay is not a perfect world, sometimes it benefits the buyers (when sellers don't know what they're selling and one canny buyer knows its true value and noone else notices).
But sometimes it benefits the sellers (especially when the buyers don't do their research).
I present exhibit A, the Voigtlander Nokton 40mm f/1.4, brand new at B+H, US$450 for either Multi- or Single-Coated versions.
Now let me present exhibit B, the exact same lens brand new, sold for €440 on fleabay (US$580). That's the most expensive one that's actually sold, but there are some listed for US$800 up there.
Again, it's a lens I would like, but I'm not paying more than $300 second-hand. I may eventually give up and buy one new from B+H. Or buy two, and resell one on fleabay for more than I paid at B+H.
If you can't beat 'em, join 'em.