Eventually I figured out how to get some of this information from eBay. Here's the graphic I created that shows how to do this:
Step One: type in your search and hit the "Search" button. Step Two: Click "Sold Listings" on the bottom left menu. The page will reload and eBay will show you sold listings instead of current listings. You can get a pretty good idea of what people are willing to pay based on these sales.
So what can you do if there are no matches in current or sold listings? Well you can try searching for a more generic version of the item you want to sell. That's not always a good option. In this situation, comparing to other Pyrex patterns wouldn't be useful information.
When I strike out on eBay I turn to Worthpoint. This is a paid service so it's probably not worthwhile for the casual eBay seller. The cost is about $20 a month. In return you get access to several years worth of price information from eBay and other online auction sites. You can find most products in their database of 300 million records.
The final option if neither of these work for you, go with your gut. What would you pay for this piece if you wanted one for yourself? Ask a few friends what they would pay if you like and form a consensus. In my family we tend to be pretty frugal. I figure if I'm willing to pay $50 for something, odds are pretty good that other shoppers would too!
Just be forewarned - if you are an avid collector your opinion probably isn't terribly valid. Collectors have a tendency to significantly overvalue their collection based on what they paid, not on what the current market will bear.
