Price check...

is it wired or wireless? What mavic wheel?

It's wireless, and he told me what wheel but I can't remember......he was touting it so it wasn't bottom barrel.

For wireless almost new that is a good price, I'd nab it. I did get a wired powertap mavic openpro for $300 with about 3000 miles on it that has worked well for me.

It's good to go with Garmin Edge 500?

Just got similar deal, similar price. I'd do it again!

PowerTap will sync with Garmin 500 (or my 310), but I got the GPS Joule, so I started using that.

Definitely.

Sounds like a good deal to me.

Wheelbuilder is selling a brand new G3 hub built into a DT swiss 465 wheel for $794.

If you aleardy have a Garmin 500, then the Joule won't do you much good...unless you're planning on selling it.  But unless it's the Joule GPS, I don't know if there is much of a market for it.

The most commonly noted differences between the G2 and the G3 is that it's lighter and stiffer.  The G3 shaves about 120g off the old G2 Pro+ hubs and is almost comparable to a standard rear hub built into a training wheel.  But to me, the biggest difference is the following...

"Before the G3, you'd have to send the whole wheel back to Paligap if the electronics needed any work - expensive in postage. With the G3 though, the electronics have been moved to the outer cap where the batteries go, so you can just pop that in a jiffy bag."
http://road.cc/content/review/61947-cycleops-powertap-g3-alloy-wheelset

They also sell a "dummy cap" so that if you do need to get the internal electronics serviced, you can still put a cap back on so you can ride the wheel without the PM internals in it.  So if your PT rear wheel is the only rear wheel you own...you can still ride.

Of course, when you buy brand new, the warranty is guaranteed to be intact...but Saris/Cycleops customer service tends to be top notch...and I have heard many stories where second or third owners of their products still were able to get good customer service.  I won't promise that they'll honor the original warranty...but I've heard of it happening.

So to your question...I'd say a used G2 with a Joule is a good deal...but by no means something you should jump at right away...especially if you don't plan to use the Joule.