Best small, desktop speakers, $500 or under (new or used)

The Elacs and Wharfedales are great; I mentioned them.

I’ve seen them for sale Used so your argument doesn’t hold up

MAudio BX5s while powered, are a great little desktop monitor. Put out a great tight image with impressive low end. Plus, you can add a sub to them if you want. Great bang for the buck for a powered monitor.

Many more good suggestions — thanks!
Wharfdales have a down-firing woofer — that’s helpful to know, because the rear of these speakers need to be about 1-2 feet max from a wall.
Will look at again at M Audio and Audioengine.
@axpert: environment: small office, about 11x14 150 sq ft., 7.5 ft. ceilings; nearfield listening, 2-3 feet, speakers to ears; speaker rear to wall would be 1-2 feet max. Type of music preferred, classical, jazz, but rock too. I.e., varied.
Go listen to a lot? In the time of social distancing? No. I am content to buy a good speaker or find a good value based on reviews or used. This is for listening to music at a desk. I can’t get wrapped up in comparisons, though trying 2 pairs could be fine. I can learn to like a lot of things for office listening.
Why do people sell speakers? Many reasons. Doesn’t worry me. Speakers are a very stable technology, so this seems a very low-risk purchase.

Let me add a detail that may be helpful. A friend lent me some Paradigms once and the Minis were too big and the Atoms were the right size.
Dimensions here made a difference. Viz.,

PARADIGM MINI
H 13.4 x W 8.3 x D 11.4

PARADIGM ATOM v.6
H 10.6 x W 6.7 x D 8.7

I have to add -- reading the reviews of the AudioEngine and thinking more about some suggestions here to clean up the desk area -- the Audioengines are enticing. I could be persuaded to give that Adcom stuff to my kid...

For office desktop I would go powered speakers to clean up the area, less wires as well. What are you using for a source? Some like these Fluance can connect to your computer with USB. 

https://www.fluance.com/powered-2-0-bluetooth-active-6-inch-bookshelf-speakers-white-walnut#!prettyP...