I wouldnt offer more than 50% of the new price for used.
You can buy them at Musicstore Koln (maybe also Thomann), they offer three year warranty and to my country free delivery if you order above 100 euro.
Can you try them first? If they work and with that price you have a good chance of some years of good service to your music.
I like them, there's a lot I can learn about mixing before these hold me back.
And it is always wise to keep checking your mixes in cars, on consumer audio, headphones etc.