It’s safest to buy a Dali that comes with an authentic LOA from Albert Field/Dali Archives. Field knew Dali and was a world famous Dali expert. He provided LOAs for many Dali works, often placing a Dali Archives stamp on the work itself. Though Field died a couple of years ago, The Dali Archives still exists.
Field wrote the standard Dali guide The Official Catalog of Graphic Works of Salvador Dali. This book is used and referenced by museums and major auction houses like Christies and Sotheby’s. This in an important book for the collector, as it discusses common fakes and lists the Dali prints that are known to be genuine. The collector can check on his or her own to see if the for sale print is ‘listed’ in the book.
Unless you’re a Dali expert yourself, it’s important to purchase expensive origial Dali prints from a reputable and preferably established seller. A good seller is knowledgeable in the area and has a good authenticity/return guarnantee. For the collector who wishes to someday resell the item, it’s will only be a plus to have the receipt or other documentation showing it was purchased from a known reputable and knowledgeable seller or auction house.