I'm sure there is a very easy answer to this.  I want to ask a requester a question about a requested condition before I send off the book?  HOW Can i send a message to the requestor?  
Janie

If you've accepted the request there will be a PM button.  If you haven't accepted there's no way to contact them.

You would have to accept and then PM them and then decide who should cancel. This is why I don't PM over RCS. Either my books meets the RCS or not. If I can't tell from your RCS then they aren't clear enough.

If you decide to accept and PM: you need to put a deadline in and either say you are going to send it or cancel it if you don't get a response. Some people have limited computer access and don't get online everyday.   Now you have to decide who cancels. If it's a WL book you need to cancel. When you repost the book it'll offer it to the same person again and you need to decline for RCs to get it to the next person.  If they cancel then they lose their place in the WL line.  If it was a book posted to your shelf and has a lot of other copies posted: then they need to cancel.  When they reorder the book, again it'll come to you and you need to decline for RCS.  If you cancel a non-WL book you lose your place in the FIFO line.  Not a big deal if there aren't many copies posted but a big deal if there's a lot of copies in the system.