Definitely try the shoes first and I would strongly suggest you don't buy them off of ebay unless or until you get used to the differences in the shoes.
My feet have been in flux since I had a baby amost a year ago. I was a 9 in most shoes and a 9.5 in most european designer shoes. Now, I wear a 9 to 9.5 in non-designer shoes and a generally a 10 in designers, including CL's. I did have to take a 10.5 in the patent 70mm pigalles. I got a 10.5 in the metalika booties, but they are a bit tight so I probably could have taken an 11. Based on trying on the shoes you can get a feeling for your size in the different types and heel heights of CL's.