As well as the general condition, the main area to look at is the seals, there are 3 in total 1.Breach. 2. Piston. 3. Buffer. 1 & 3 are easy to do and the 2. requires the pistol to be stripped to remove the piston.

Look for Green, Blue and Red seals. If the piston seal is an off white, it is likely to be original and will fail at some point. If you look through the breach seal you can see the piston seal (see photo)

The buffer seal, not shown in the photo is at the barrel end of the breach and is easily replaced if required.

https://live.staticflickr.com/65535/...249e96e1_k.jpgDSC03187 copy by Mark Davis, on Flickr