It's a private transaction between the owners involved. WorldMark handles the actual transfer based on written instructions from the seller, but the terms (price, method of payment, etc.) are handled privately between the buyer and seller.
I have rented credits through the WorldMark forum,
http://forums.trendwest.com/ubb-threads/ubbthreads.php, quite a few times with no significant problems. You need to be registered on the forum as an owner to access the credit rental section. Some of the renters there are also new to the process and need some hand holding.
In additon to the forums, some of the WorldMark resellers offer one time use credits. If you are concerned about a private deal via the internet that may be your best option. I've gone that route too, although the prices tend to be a little higher. I'm not sure if posting the resellers link would be a violation of the TUG no-advertising policy. Send me a pm if you want that info.