Honestly I think that's a hard question to answer on 2 principals:
What are you willing to accept, and
Do you know what you want?
I understand that you are aware of the model you're looking for but do you have specific details about that gun? Upgrades, impacts, etc...
Unfortunately while here, I've found that the biggest thing to help you in your task is the search button in the top right.
Again, not trying to be a jackass, but a LOT of this has been covered before. It's not that people don't want to help, but it's kind of like answering the same question 100 times. It gets old lol
But to answer you more specifically: Look at the parts that are important. Are THOSE in good condition... can you even tell? Does the person you're buying from have feedback and if so, what's been said about them? Ie: Can you take their word.
Look up AEG on Google and look for inner workings and try and get an idea of what is GOOD in an AEG and what's not good, etc...
Hopefully with a little time you'll be able to answer your own questions until you're really stuck