I am doing my first deal in the bazaar.  I state in my post that to get the deal, please post what title you're ordering and what you want your freebie title to be, both in that thread and in a PM.  (That seems to be the standard way.)

I've gotten a few orders, but I've also gotten orders for two books from two different people who didn't contact me either way, and of course paid two credits for the books.  So either it's a coincedence that these orders happened, or maybe they are expecting me to buddy back a credit?  I'm not sure.  And I'm not sure if I should PM them and ask.   My thinking is that if they didn't follow my instructions, should I really chase after it?

Did they post in your thread stating they were taking the deal? If not, I would say that those 2 orders were just regular orders that just happened to come through while you were running your deal. If you are looking to get rid of some books, you could PM them to explain that you are running a deal so they can take a second look at your bookshelf.

Like Shelly said... if you don't have a PM or a post from them, it is just a regular order and no action is needed on your part.  However, I would not send them the 2/1 special notice...because then you wait and wonder if they will want an extra book... and  you might hesitate to mail quickly.  I would instead send them a PM offering them a 3/2 deal and say to PM you before you mail if they are interested.  If there is no response, just send the book whenver you planned to.... if you do get a response... you get 2 credits for 3 books... yippee for you... and free book for them.

it is likely they ordered the one book and then was perusing your shelf and saw another book they wanted. 

I'd just send them the books.  If you wanted to be nice you could offer the 2 for 1 to them but you don't have to!