There are only a few sellers I'd trust on ebay but if I were you I'd buy decants from people on this board because you'll get them cheaper than ebay and I trust the people on this board more. Theres a few ways you can do this:
1. Go to the swap board and look through peoples listings
2. Look up the fragrance you want in the directory (hint: when you use the directory do a search in the top right where it says search rather than in the space where it says "Fragrance Name". For some reason if you do a search in the Fragrance Name line some fragrances like Millesime Imperial don't show up unless you put all the proper accent marks above the letters. I need to ask Grant to change this.) Once you find the fragrance you want go to its page and scroll down to where you see Basenotes Market Place and it will show who has that fragrance on their wishlist, swaplist and who is willing to decant. Pick some of these members and send them a message and most will be happy to help you.
3. Go to the Fragrances Wanted board and list what what decants you are looking for and other members will see what you want and should send you a message within a few days.
Welcome to the board and good luck!