I wouldn't pay more than $250 for it. Probably has dead batteries. Most used bikes with lead acid have dead batteries because the user doesn't want to fork over $$$ for new ones every 1-2 years.
And is it worth it.. i would say no. 24v motor, lead acid, brushed, and 73 pounds.. that is actually worse than an eZip trailz bike that you can buy new for $400 at wal-mart.
If you have $500, it is better spent on a variety of kits for sale here on the forums and elsewhere. You'll have money left over to go towards some lithium batteries that will last longer than 1-2 years and the thing won't weigh a ton and a half.
Seriously, if you buy a cheap motor kit like the yescomusa, cell_man, etc. kits for ~$200, and add a lithium battery to the mix, even a cheap one like a ping, your bike will be sooo much better than this.