Harry Potter has proven to be a hugely popular film franchise, so it can be a minefield when looking for authentic signed items. If you are looking for the harder to find signatures, then do be very careful when buying J K Rowling and multi signed items by the top names, as these are very widely forged.
And do avoid the multi signed posters so often offered by sellers in the USA (who are often not based in the USA at all, but some other country!), as we have yet to see one of these that is authentic. Just try getting ten signatures of actors from any film on a poster at a premier, and you will soon see the problems you will have, as all you will end up with is a crumpled mess!