I agree that $5,000 for a used WR250X (or WR250R) is nuts.
For that money and for me to want to buy it, it would have to be a 2017 model that was barely used.
In my opinion, a fair price for a 2008 WR250X in great condition is about $2,500.
I'm sure that a lot of guys selling these bikes would scoff at that one, though.