I've sold pre-entries a number of times, and I suppose buying is about the same, only different.
The seller writes a little note saying "I, (seller's name and maybe vehicle number) hereby transfer my entry to so-and-so (buyer's name and maybe vehicle number).
That's all. No mention need be made of any exchange of money. That note is handed to JoAnn or Warren or someone in the BNI trailer and entered in the official paperwork. The buyer will need to have a medical form on file -- might have to fill one out on the spot if he's really new to being there (instead of having filed by mail).
That's all there is to it. The buyer gives the seller whatever remuneration has been agreed upon and both are happy.