I am sorry, but I don’t understand why you want to protect the preview? Anyone can purchase your item and get the full sources....and some of the files don’t even require to source code to be changed in order to work. like those banner rotators.
It seems it is impossible to fight theft. The only way is to keep the tracks of stolen items. You can place the “invisible” unique text within the preview (for example “My super project xyz”) and then use the google search request “My super project xyz” filetype:swf to get the list of all sites which are using your file.