I think the key fact that everyone is skipping over is your pricing. You mentioned that you are selling domains for $500. Most people are not selling domains for $500. Take away the registration costs, renewal costs, time spent researching the names, acquisition costs, managing costs, taxes, etc and it makes very little sense to sell for $500 or even $1,500. Especially when you are considering the fact that a business is gaining an asset, a brand, a virtual office, etc.
Regardless, given the economic uncertainty, sales in the lower pricing tiers are increasing. So people may not jump on that $x,xxx or $xx,xxx domain as quickly, but a $xxx is not much of a risk for them. Additionally, there is always an increase of bigger investors taking advantage of people underpricing their assets. So when the economy is down, those cheap assets get bought up quicker than those that are priced higher. Just basic economy.
Why is this happening in Sedo for you more than Afternic? I have no idea. But my guess is that Sedo has a better system to browse for names than afternic does. For the most part, Afternic names are getting discovered through type-ins. Where in Sedo, there's a small chance someone is using their search.
Personally, during the last 5-10 years of using Sedo, I have seen no results. Everything from their Great Auctions to their regular listings has been a complete waste of time for me.