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.