private torren sites still get thier files from somewhere.... it's the torrent protocol that can also be monitored.. meaning, most use common ports to transfer files. and ISP's even sometimes monitor those ports, and try to catch you (if it's a common port). If you use a different port for the transfer protocol, it might not catch you that way. In regards to being on a 'closed' board, there's nothing to say (if they are popular) enough that you can't get on thier network or be a member. It only takes one idiot to spill beans, and have someone log on and 'infect' files from that site. If the site mods are smart enough, they will catch it before it transfers...
nothing is really safe or anonymous. The only thing about using a pay service (which can still be tracked to a paypal account), is it uses http protocol to download, so it's a common port for downloading. They may have a download/link history but I've never heard of problems with that. If that's the case, then download for 30 days (if you pay), then maybe get another account, but it'll still be all tied to the same payment system.
If you download for free, you have to wait between downloads depending on the time, etc... and of course could take a while depending on file size and quantity of files. If you pay, you can transfer up to 5 gigs per day or something crazy... and the servers are much faster...
regardless, that's the best i can do to help. LOL. i'm in no way benefiting from telling people about rapidshare. the other thing regarding TOR, i think you might be able to setup a torrent software (not sure which ones are compatible) within the tor network, which masks your IP very well, but your speed of transfers will drop dramatically, even though most of the world uses high speed connections now.