Concerning the operational details of a 2 for 1 offer, I figure...

1. I publicize a 2 for 1 deal. 
2. A "buyer" orders one of my books, and then PMs me with the name of the other book he/she wants.
3. I ship both books.
4. I get credit for the one book and remove the other from my bookshelf.

Is that how it works?

Yes. Just make sure to take the book they PMed you about off your shelf.

You got it. 

When you start a thread, and are listing your conditions for your deal, remind folks to both PM you and post on the thread, both the book they ordered for credit, and their freebie pick, so people know which books are taken, just in case you can't get to your computer for a little while, or there's a hiccup in the system, and the picked book still shows up on your shelf, or any number of reasons.  If someone sees a book on your shelf, but there's a public post in your thread about it, they wont order it.  Folks around here are very nice and respectful about that sort of thing. 

Ask away, but you may want to look around the book bazaar and see how other folks are offering deals, how they do their conditions and such.

There's a sticky in the book bazaar with infomation on this with a couple posts on tips for doing deals in it, too.

You can also put it into your signature line.  If you make regular posts to the forums more people will see the deal you are offering. Don't post deals on the genre page you prefer.  Those are for discussing that genre and not for advertising your bookshelf. But you can put it into your signature.