Here is some advice for buying an antique corkscrew, assuming you are not just a collector, but actually intending to use a beautiful piece of equipment. These unusual corkscrews make fabulous, and individual presents for people who like their wine!
The important thing to look at is the pitch and thickness if the screw. Here are three different ones.
The one on the left has a delicate thread and a long pitch. It is easy to use, but it does not grab a difficult cork.
The one on the right is too thick. This is typical of a modern corkscrew. It is the least effective of the three.
The one in the middle has the best compromise. The pitch is good and the screw has enough throw / material to grab a difficult cork. Note that the steel is flat and not round like the modern one.
If you are interested to know a whole lot more about antique corkscrews, go to the Corkscrew Collecting site.
Where to buy an antique corkscrew
Try also Ebay and Etsy.