When buying used, you are giving up a lot. You have no warranty or customer service. But you also have a rifle that has been tested. Does this gun come scoped? Because I would say if you are getting this outfit ready to shoot and the seller is honest with you then 400 is a fair price. If you are buying a used gun and then have to spend 150 dollars on a scope and rings, then your price is probably similar to what you would spend for a new one (pyramid air package deals can get you in this ball park with a pump included). At this point the scales are balancing. But if you then have issues with your gun, like it was tuned out of whack or you have a leaking seal, then you are going to be spending more money just to get it to what you would get from factory. There is always a higher risk when buying from a private seller. But the risk is weighted against the potential for saving money and getting a 'deal'. I think it all comes down to your budget. If you are honestly going to be okay if things go wrong (needs repairs, or it has a trashed barrel because someone cleaned it improperly, or it has rust hidden someplace, or the gauge sticks, or or or or or etc...) then go ahead and roll the dice. Most likely it is a good solid gun and the person selling it is asking a fair (if a little high in my own book) price. But if your budget is tight, and you are making sacrifices in other areas in order to invest in this rifle, then I would advise purchasing new from a dealer. The slightly higher price you will pay, or even premium price, will go to a dealer that is focused on making sure you are happy with what you get. You will be protected against defects in manufacturer, you will have support for your equipment, and (most important for me) you will be ensured that your equipment was not abused by someone who was careless or clueless. When you get more into rifles and know your way around a system you will feel much more comfortable working on them and buying used. But if you are new to PCP's it might be good to start with one that is factory example and "standard".
Probably not what you wanted to hear but hope whatever you get you enjoy it and shoot the heck out of it. Have fun!