Best small used monitors under $1000

Kef ls50 lack dynamics and need expensive amplification.

Trust me - Jbl lsr 305 - very good dynamics that some 10k speakers are unable to deliver. The Neumann kh120 is 10% better in sound quality, better build quality (cast aluminum), metal grilles, made to last forever!
chord mojo DAC - the very best if you listen to low Bitrate stuff like webradio, Spotify or MP3 

Dynaudio DM 2/6. The DM 2/8 is what I use really smooth in my opinion or the ZU Audio ZU cube are great.

KEF R101.  I have two sets in two different rooms.  I bought them new in the early 1980's.  They sound wonderful, and get better each year.  I see them from time to time on ebay and other used resources.

Dynaudio Excite X12's - great sounding small speaker.

I bought a pair of KEF LS50 used with stands for $750!  They are found used regularly for less than $1,000.  Originally I was driving them with a NAD 326BEE which I picked up used still under warranty for $275.  This makes up a pretty amazing system for around $1K.  A better option would be Rogue Audio Sphinx integrated which you could find used for less than a grand.  LS50 is least expensive Stereophile Class A recommendation and they keep a Sphinx on hand for many of their speaker comparisons.  Stereophile believes this pairing is ridiculously good for the money.