The listing is called: "Apple Newton eMate 300 Laptop 4MB Upgrade DT320 RARE K5" At ~$25 including the shipping it's a little pricey, but if you've wanted the 4mb upgrade here's an opportunity. There's 3 available right now. This is the Newer Tech one.
Note: *Not my Auction* Just pointing it out. That said, exercise caution whenever buying on Ebay as there probably are unscrupulous sellers out there.