We purchased my ring from Perry's in North Carolina (no tax). My ring is vintage Tiffany & Co. and it has been confirmed as authentic by Tiffany & Co. upon an insurance reevaluation (it's only $200 and it gives you the true value for insurance). We definitely saved on the ring, and I'm happy to say that the official value Tiffany assigned is over double what we paid! I have a BMW (lower series) on my hand!
It came with the original box, but no papers so we got it for a better deal because of that. On staff, they have a few appraisers that had been with Tiffany for a period of time. Instead of negotiating the price down, we asked for an extended return policy so we would have enough time to return it if it was not authentic.
If you tell the people at Tiffany that you are looking to buy a vintage piece, I've found them to be helpful. Tiffany actually does NOT want to interfere with the vintage and estate market, as they do not sell vintage in their stores and when pieces hold their value, it only helps the business in general. There are a few retailers and auction houses that Tiffany has certain programs with.