I probably should leave this to Daniel or Basil Beard to answer for you, but even they will need a couple of more details to be of much help. Are you wanting to sell(download) the file IN the board, or sell Access to the board? If you selling access, then I suppose you could give the user individual perms onuse() and revoke it via Display.php/Display.template.php after a download, but that could be complicated. .ooO{I think anyway}
If I might offer a suggestion; why not have the board access public with all the avaliable file "Descriptions" (no links) posted along with their credits cost so that everyone can see what great stuff you have for sale. Then the item's onuse()-->getUseInput() routine becomes a simple matter of populating a <select> from the dir containing all the files and the member simply selects the file to download. This way your transaction is handled withing the scope of the Shop without fiddling with the code in "Display.php or Display.template.php" in order to remove individual perms after the download, kapeesch?
:-) Hope it helps
Phydeaux