Art is worth what somebody will pay for it, however when it comes to buying Dali, especially on ebay, it pays to know a few simple rules:
1. Dali signed some series and didn't sign others; beware of the seller with all handsigned dalis and no unsigned/ plare signed
2. Dali signed nothing after 1980, luckily in 1980 ARCHES paper changed forever and included an infinity sign in its WATERMARK, thus any Dali with an infinity watermark and handsignature is a fake
3. If you are paying big money for Dalis, first buy ALBERT FIELDS REFERENCE BOOK, it will save you a lot of money as it has 40 pages of how to spot the thousands of Dali fakes which seem to find their way onto ebay.
4. Often the rarity of the Dali is not that it is handsigned, but lies in the rarity of the paper, with correct watermark, in fact the more dali signatures on the paper the less likely it is to be authentic!
I am happy to help investigate pieces that you doubt, best wishes