[Freon]

One tire being measured at one spot at 6/32" doesn't mean much. You need to have careful standards when you decide how worn a tire is. That's not a determination you should leave up to the seller or base off a single picture of a treadwear gauge.

Definitely be wary of buying used tires. I'm not saying it's always a bad idea, but it's not a great exercise for buyers who don't really know how to inspect a tire.

I would never pay more than 50% of new price for a used tire, even if they only had a few thousand miles and were in excellent shape. $500 sounds steep even if they're in excellent shape with 6/32 measured in multiple spots IMO.