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

I picked up new Kef Q150s last month when they were on sale half price for $300.  Some dealers may still have some at that price. 
They are very good. I rotate three sets of bookshelves at my office for variety. $2.2k msrp Spendor SA1’s, $600 msrp Kefs, and $300 msrp NHTs.   
The Spendors are the best, but not that much better than the Kefs, which themselves are much better than the NHTs. I’d say the Kef’s give me 95+ percent of what the Spendors do at 28% of the cost (14% of cost if you can still get them half off). 

These work really well for desks as well.  They decouple and isolate your speakers from the desk and tighten up image focus and bass response.  You can angle them or raise them also, whichever you prefer 😀

https://www.guitarcenter.com/IsoAcoustics/ISO-130-Isolation-Stands-for-Studio-Monitors-Pair-15000002...

Probably go JBL 530, Paradigm Atoms, or KEF LS50

I’d make an offer on these:

https://www.audiogon.com/listings/lisa2h65-clairaudient-the-one-monitors