I bought my Excel on ebay over two years ago. I had the same thoughts as you. I checked the seller's feedback as well as e-mailing the seller to kind of gauge the person. They were very nice!
The auction also contained pictures of the serial number, which put me at ease a little more. However, when it is all said and done, ANY auction holds the same potential for scams or losing money. As Larry said, Paypal provides a little more security, as does your credit card.
I suppose, if you found one you like and know of someone who lives in the seller's area, you may be able to set up a viewing. The good side is, my Alembic was not only perfect, it also arrived in tune!
Paul TGO