We just bought in the office a second hand Zortrax M200 from 3DPrima.com.
We started testing it and we noticed a small stringing effect and also a strange shiny effect on surfaces - even if the objects have good proportions and respect the model dimensions they look "melted".
We already have another Zortrax and even after 1 year and a half the quality of the prints is still amazing.
Can this be solved in any way or should I return it?
Sounds like it could be a temperature issue. Accessible components affecting temperature are the thermocouple and the thermocouple amplifier circuit above the extruder stepper motor. The A-to-D converter in the CPU can also affect it, but this is not user-accessible other than by replacing the whole motherboard. You can test the hypothesis by lowering the extrusion temperature using the new Z-Suite, or with a third-party hardware temperature control.
One of the fans was not working. There is still a little stringing compared to the other Zortrax, but much better. I will try to change the thermocouple also.