Pour le problème de message "Impossible de finaliser votre achat, Impossible de vérifier le fichier de distribution du produit. Il est peut-être endommagé ou non signé", cela signifie qu'il y a un fichier endommagé, il faut supprimer les fichiers temporaires, voir procédure décrite à ce lien (en anglais):
https://discussions.apple.com/thread/2710174?start=30&tstart=0
En bref, il faut:
1) Ouvrir l'application Console
2) Relancer la tentative de téléchargement
3) Le message d'erreur apparait, il faut alors regarder dans la console un message du type:
8/31/11 6:50:37.121 PM App Store: FRPurchaseManager: Preflight operation for 415571499 failed with error: Error Domain=com.apple.appstore Code=0 "The product distribution file could not be verified. It may be damaged or was not signed." UserInfo=0x7f9b2464aa00 {NSLocalizedFailureReason=Cannot create PKProduct from "file://localhost/var/folders/3p/sflvgxwj3hs4kdkjpgb30r4c0000gn/C/com.apple.app store/415571499/preflight.pfpkg" Error Domain=NSURLErrorDomain Code=-1100 "The requested URL was not found on this server." UserInfo=0x7f9b2464ac00 {NSUnderlyingError=0x7f9b24648230 "The requested URL was not found on this server.", NSErrorFailingURLStringKey=file:///var/folders/3p/sflvgxwj3hs4kdkjpgb30r4c0000gn/C/com.apple.appstore/41557 1499/preflight.pfpkg/index.sproduct, NSErrorFailingURLKey=file:///var/folders/3p/sflvgxwj3hs4kdkjpgb30r4c0000gn/C/com.apple.appstore/41557 1499/preflight.pfpkg/index.sproduct, NSLocalizedDescription=The requested URL was not found on this server.}, NSLocalizedDescription=The product distribution file could not be verified. It may be damaged or was not signed., NSUnderlyingError=0x7f9b2464ac40 "The requested URL was not found on this server."}
Et noter le chemin indiqué dans NSErrorFailingURLKey=file://
/var/folders/3p/sflvgxwj3hs4kdkjpgb30r4c0000gn/C/com.apple.appstore/41557 1499/preflight.pfpkg/index.sproduct, NSLocalizedDescription=The requested URL was not found on this server.}
4) Effacer le répertoire /var/folders/<nom> indiqué dans le message d'erreur
5) Relancer le téléchargement