because the poster sent me a description of the book after she had accepted the request, and it isn't in very good condition, if I request the book again will the same book be offered to me?

It depends. If you were #1 on the WL and now you cancel, then the next person will get the book, and you'll lose your spot in line (but will eventually get a different copy). If you requested the book in something other than FIFO order (say, if you requested an extra book from someone's shelf when you're already getting a book from them, or ordered from a particular person's shelf), then requesting straight will probably get you a different copy.

But I'm guessing you just sent a generic request for a book that was available in the system FIFO. Unless someone else happens to request the book between the time you cancel and re-request, I think you will be requesting the same copy.

If the book isn't in postable condition, the poster should be the one to cancel in the first place. If it is in postable condition but the poster just contacted you to let you know about a less-than-stellar condition and you'd rather hope for a better one - well, I guess you have to decide whether it's worth this annoyance (not knowing when to re-request the book) to avoid this particular copy.

Why did the poster send you a pm?  Either the book is postable per PBS guidelines or it isn't (unless there's an RC involved). 

If you cancel the transaction, you loose your spot FIFO and the book is offered to the wisher behind you.  If you cancel the transaction and there is no wishlist, yes there's a good chance that the book will be offered to you again.  This is why it's the sender's responsibility to cancel the transaction if the book is unpostable. 

If the book is postable, there is another method which is for the sender to cancel the transaction.  Then you add an RC to your account, the sender reposts and declines your RC, and thus the book is offered to wishers behind you (and you can remove the RC).

Since the sender sent the PM, it seems like it would be their responsibility to cancel since they started this mess whether the book is postable or not.

When they cancel, if the book is not wish listed your request will be passed on to the next person in line.

If the book is postable, then they can repost it. If it is a Wish List book and you don't want it to be re-offered to you, set up a RC saying you don't want the book in the condition they said theirs was in so they can deny that and get to the next wisher.

If the book is not postable, they should be cancelling and not re-posting.

I had this same situation happen.  Got offered a WL book with 0 offers and I'm the only wisher.  Someone posted it then PM'd me pictures of the cover- the top of the cover is messed up.  I said I don't want it but it hasn't been canceled by the poster yet so it looks like it's going to time out and I'm going to lose my place FIFO.

Amanda, if it's messed up in a way that it's not postable and they won't cancel it I would contact TPTB. If you're the only wisher I wouldn't worry much about your place in line :)