Most organizations would just toss this stuff in the trash bin.
Yes, and they add to the waste and inefficiency problem in the world.
Nothing wrong with passing on these items as such.
However, there is one aspect where I think they're being disingenuous here. Under their 'About us' section it gives the purpose of the site:
The purpose of this website is to sell construction equipment, tools, and materials that were purchased for and used in the recent construction of the World Headquarters of Jehovah’s Witnesses. This is in harmony with the spirit of Jesus’ words at John 6:12, "so that nothing is wasted".
But in the context, John 6:12 is about gathering up the excess loaves and fishes, which Jesus had freely provided, for later.
A principle the org often likes to quote is at Matthew 10:8 - "You received free, give free". I don't know how many of the items they're selling were donated, but even the ones bought by the org would probably have been purchased using donated funds originally. So shouldn't they be giving these items away, like Freecycle (with perhaps just a nominal postage and packing cost where applicable)?
Ok, if you wish to sell them as second-hand items like eBay, just do it. Don't make some tenuous link to the scriptures to try and justify it! Just a thought.