Some things to check:
Make sure the serial number on the receiver and barrel match! It's mainly a big deal for black-stocked AUGs as they are rarer...but if it's got a replacement barrel, it's good to know. Factory guns have matching numbers.
Check the bolt sleeve when you get home...the older design can crack. A spare parts kit can swap out that part, though. (It happened to me)
I have two...one 902SA, one 906SA. I love 'em both. Very fun, easy to clean, accurate as I am (as much as I can be with a donut recticle)...
The items you refer to as gas plungers are basically springs. The gas system is housed in the barrel (off to the right side)...the gas piston is there. It pushed back on the right "plunger", thus recoiling the whole system.
$3,000 is definitily in the ballpark, and possibly on the low side if it's in real good condition. Check the gas piston, and see if there are any "brass dings" in the stock (you'll see little gauges just behind the ejection port)...if the internals feel a little "tacky" (not worn smooth), it's in pretty damn good condition. The trigger pack probably won't show any wear (the one in my 902SA doesn't, and I know it's been pretty heavily used by previous owners)..so I wouldn't worry about using that as a gauge of usage. In fact, the only way I could tell it was heavily used was the internal parts smoothing thing, and the wear on the bolt carrier, with the gas piston (it's face is very black..half an hour scraping at it with a brass brush helped, but it's still pretty discolored).
I have a 14" commando barrel I had a Phantom permanently attached to...it's pretty loud, but woo! Talk about a small package.
For the price they're asking..I, personally, having two, would consider picking up a 3rd.
Get it, you won't be sorry. And if you are...you can sell it for an instant profit. No worries