what do you do when someone waits to send your books until they've received theirs? Do you leave a comment about it, mark positive or neutral or just let it go.  And how common is that ??

First, I'd like to point out that I am not a BoB member, so realize that when taking my opinion into consideration. 

This isn't right.  If everyone waited until they receive books before they send out books, then no-one would ever receive books.  I understand that perhaps the member has been burned in a bad trade in the past, but this (waiting to send out books) is not the correct response/solution.  I have heard of members that try to do this, but every time I've heard it, it has been against BoB trading policy.  I would try to recommend to the other Sender the correct policy and if they refuse to follow it, leave a comment warning others about their violation -- that's what the Feedback is designed to help protect against.  I would also take it a step further and send a note to the site administrators via Feedback about this member's BoB trading habits.

I would definitely not "just let it go".

If everyone waited until they receive books before they send out books, then no-one would ever receive books.

This is really common with BoB members, and one of the reasons I stopped using it. Well, that plus the fact that I already have more unread books than I could possibly finish in my remaining lifetime. :)

It made me furious and I thought about withholding my own books just so I could experience the paradox.

I don't use BoB, but if I did and that happened to me, I'd leave a neutral with a comment.

There's no need to punish current users for past users mistake.

Do "boxers" get any kind of referral bonus for getting others to sign up? I got a random message saying someone had recommended me for BoB. My first thought was that she liked what was on my shelf, so I thought about messaging her offering a deal, but when I got to her profile, I noticed we had a VERY low similarity percentage, so I didn't bother.  I'm still just kind of wondering if it was a random referral she sent to a bunch of people or if maybe she did find something appealing on my shelf.

Stephanie, I can't answer your question about referrals, but I wouldn't put much weight in the similarity percentage.  Many of us have books posted from multiple sources and/or order books for multiple people.  I have no interest in fantasy and science fiction but frequently order it for my grown children.  I would PM the person acknowledging the BoB recommendation and asking if they were interested in a deal.

Like sasssy25, I have no idea about the referrals, but do agree that the similarity percentage as reported by the site isn't worth much.  I'm not sure how it's computed, but I've checked out the wish lists of a few people that the site reports as having a high similarity to me--most of the time only a few books on their wish list are books I'm interested in.  On the other hand, some people that the site reports a low percentage of similarity to me (in the 30% range) have quite a few books on their wish list that I'm interested in.