Got my first journal entry today for a BookCrossing book I sold on eBay:

http://www.bookcrossing.com/journal/2898772

I didn't sell it to make a profit but to advertise BookCrossing, therefore I listed it with a fixed price that only covered the listing fees and shipping costs. In the auction listing and on my eBay "me"-page, I explained the BookCrossing concept. I've sold two BookCrossing books this way, and plan to list some more.

I figured that if you're looking for used books on eBay, you're likely to be

a) a reader
b) interested in finding free books
c) internet savvy enough to journal your book

and thus an ideal candidate for BookCrossing!

I would never sell books that were registered by another bookcrosser, of course. But I think it's a great way of introducing new people to the BookCrossing concept. I also noticed a significant increase in tell-a-friend referrals while the auctions were listed on eBay. So even if no one buys the book, at least people are directed to the BookCrossing site.