So much depends on the market and lots of small details. Tanks can be good or bad even when cared for if the person doing it is careless about the way they scrape algae. Check the silicone.
Stands can be made well or not worth hauling home. Lights can fall into the same category.
So the real price has to be what you feel it is worth to you personally versus what the seller feels is right. If you really, really want it, it will be worth too much. But if the seller really, really wants to get rid of it so he can move, he might be willing to give it away.
For that tank, $500 might be right in my area but the time of year changes that as well. People move when school is out and when they move they sell things cheaper.