Hi,
I wrote little script that make my life with Rockbox easy

It does three things:
1) Create playlist from marked songs and put it to song's folder
2) Move marked songs to separate folder( /<source-folder> (b)/ )
3) Delete files in folder before last marked.
Every thing has a choice to delete source .bmark file if you already don't need to keep it.
I tested it on Sansa Clip+, but it must work on other platforms.
Some explanation.
1) For example you have collection of songs
/music/MegaHits2013/*.mp3During listen you bookmarked few songs you liked. By default bookmarks saved in
/music/MegaHits2013.bmark file.
After you run the script, chose "Create playlist" and select
/music/MegaHits2013.bmark file you will have an
/music/MegaHits2013/#best.m3u8 file with all songs which you previous marked.
2) For example you have the same collections of songs
/music/MegaHits2013/*.mp3 and
/music/MegaHits2013.bmark file.
After you run the script, chose "Move to new dir" and select
/music/MegaHits2013.bmark file you will have created new folder
/music/MegaHits2013 (b)/ and all marked files will be moved to this folder.
Usually after this I go to file browser and manually delete source
/music/MegaHits2013/ folder to leave only favorite songs and free space.
3) For example you have an audiobook saved as
/audiobooks/Erich Maria Remarque - Three Comrades/001.mp3...
/audiobooks/Erich Maria Remarque - Three Comrades/154.mp3During listing you save your last listed position (ex. 107.mp3 03:25) in bookmark so
/audiobooks/Erich Maria Remarque - Three Comrades.bmark file is created.
After you run the script, choose "Delete files before last bookmark", and select
/audiobooks/Erich Maria Remarque - Three Comrades.bmark file, next files start from
001.mp3 till
106.mp3 will be deleted.
Be careful if you have files named
1.mp3,
2.mp3,...
9.mp3,
10.mp3 and numerical sorting setup in you rockbox! Because script sorting files only alphabet and delete
10.mp3 immediately after
1.mp3, not after
9.mp3.
If you have any ideas about it, please write.
UPD: Added 4th function for cleanup - "Del all except marked".
It deletes all music files in directory except bookmarked files. It doesn't delete pictures, texts or other fies, only deletes files with same extension as last bookmarked file.