PS Audio XStream biwire, what are they worth?

I think you’d have a difficult time getting someone to buy them for anything close to, never mind more than, $500 given their age, length, and the general depletion in all used cable prices over time. These cables are likely 10 years old, and no longer made so unless someone really wanted these you would probably have to deal with offers at far below that asking price. Of course, you could subscribe to the blue book and learn how these have sold recently if you want real data on this. I think the question really should be ‘can I do better than these at $500?’ and that really depends on how well you like them. I’ve owned lots of PS Audio gear but their cables are the least interesting part of their product lines to my ears. Most decent alternatives at the same price always sounded better to me. But since you own them, they originally cost a ton, why not get some use from them to establish a baseline to your ears and then trade them in on something you like better.