Well, anyone can ask $839; what you need to know is whether anyone is actually buying it at that price.
Here’s my take:
First, you have to get over the idea that, because you paid a lot for it a couple of years back, you should get a good price for it now. Two years is quite a long time in the world of computers and technology.
To tell the truth, having just done a (very) quick browse through a couple of tech sites like TechBargains.com, there’s no way i would pay $500 for your computer. For one thing, even leaving aside the specs, it’s two years old and, as Euphonious Polemic notes, that’s quite a significant factor all by itself in terms of the (potential) life of the hardware.
Within a few minutes of searching, i just found the following Lenovo laptop for $479:
Lenovo IdeaPad Z570
Intel Core i3-2310M Sandy Bridge Dual-Core @ 2.1GHz
15.6" 1366x768 LED
4GB RAM
750GB HDD
DVD burner
802.11n wireless
Win7; webcam
6-cell battery
HDMI; numeric keypad; 1yr warranty
As you can see, this computer has the same amount of RAM as yours (although te RAM is probably faster), and a larger HDD.
You computer’s processor has a rating of 1354 on CPU Benchmark, while the i3-2310M has a rating of 2548. Higher is better.
I am going to disagree with Euphonious Polemic about one thing, though: the small screen might actually work in your favor, if you happen to find a buyer whose main needs are for portability.
Now, just because i wouldn’t pay $500 doesn’t mean that you shouldn’t ask $500. If my periodic browsing of Craigslist is any indication, people frequently ask stupidly high prices for old tech gear, and in some cases they probably get it. All it takes is one sucker to read the ad, and you’ve scored 500 bucks.