The problem is used monitors dont come with massive reductions and you have no warranty. At this budget level you can get pretty good monitors from KRK JBL Fostex Yamaha Mackie Presonus M-Audio ADAM or Behringer (I would not recommend Behringer personally) and get a good warranty period. If you used monitors go bang after 9 months you got no monitors and no money, unlikely you may say but even in the used monitor market you are not going to get a massive step up the ladder.
I would start looking at the ADAM F5's, KRK RP5, KRK RP6 (over budget), M-Audio BX8, Yamaha HS5/7 (the HS7 is over budget), Fostex PM0.5
I would suggest you look into placement too, you may want to look at front ported design if you are working on a desk with a wall behind. You may also want to consider isolation pads, some people use books or other things to adjust height placement but if the desk it not sturdy or the correct height, speaker stands or isolation pads can be a good investment. Something i noticed with the KRK g3 speakers is they have isolation material on the base of the speakers to reduce vibration, I still use iso-pads, its only really useful though if the speakers are at the correct height, and I cant vouch for how good a job it does at reducing vibrations.