Thank you Jesse for the entry and Mike for the assistance!
All set.
1) Seller clicks the event whose entry they want to sell.
2) Seller clicks the edit/transfer button.
3) Seller inputs the buyer's email address.
4) Buyer receives an email to accept the transfer. if the entry is for a different category than the buyer has in their profile, there will be warning message. You have the option to write a note in there, to explain. Certainly if the seller's entry is a cheaper price than the buyer's entry price (ex: junior vs old), there may be some issue...unknown how that works. It wasn't a factor for me.
5) Buyer pays
bikereg.com a transfer fee ($10 in this case) and a processing fee ($1.80) through the site.
6) Buyer pays the seller on the side (paypal, etc).
7) Buyer contacts the race organizer to change the entry from the seller's group to the buyer's group.
8) Buyer confirms on bikereg's "who's registered?" link that they are on there, in the right group.
Besides the price thing in step 4, there may also be issues with group sizes. So, especially if you're in a big group, contact the race director first to make sure they can handle another entry into that crowded field.