I know that a lot of people have had good experiences with used airguns and relatively few bad ones. Still, I'm very nervous buying used and dealing with someone, somewhere, I don't know personally.
I do a lot of gunsmithing for friends and seen many used and abused airguns that have been "fixed" or "upgraded" by amateurs who shouldn't be allowed to have a screwdriver.
The casual "tuners" who are always looking for maximum power are the biggest concern - the damage they can do to an airgun is internal and not always apparent by buggered screws.
"Open box" deals are questionable, professionally refurbished items are probably fine.
Personally, I prefer to buy new, pay the asking price and get the manufacturer's warranty and a dealers after sales support. IMHO, unless I know the specific item and the owner, the money saved buying used is not worth the risk of getting damaged goods.
On the flip-side, if you are mechanically inclined, buying a "parts gun" or a damaged gun can be a fun project. By trading some old archery gear, I got a slightly used FX Crown that had been tuned to the point it wouldn't shoot worth a darn. A couple of hours work has yielded my favorite PCP that shoots sub-MOA at 50 yards
If gambling and saving some money is exciting to you - go for it!
...just my (paranoid) opinion.
Cheers and good luck!