I just had a request from someone who said they only accept books in "very good condition", "unless you check with me first and I approve of the condition". I would probably consider my book good rather than very good, but would like to describe the condition to the requester rather that just give up right away. How do you message someone without responding to their request?  Thanks for your help!

You can't.  You would have to accept, then PM the requestor, then, if your book wasn't accepted, one of you would have to cancel.  In that case, if it's a WL book and they cancel, they lose their spot on the WL.  If it's a WL book and you cancel, you can't post it to the next person on the WL until this person is no longer first on the WL.  If it isn't a WL and you cancel, you lose your place in the FIFO.

This is why I always, always, always, refuse requestor conditions that ask for a PM.  Besides, it says right in the Help Center: 

Your conditions should be easily interpretable and should NOT require (or ask for!) further clarification in Personal Messages

But that's just my pet peeve.

eta-except involving textbooks and cookbooks as those PMs are required per PBSwap policy.

Don't know if this has been resolved yet for you...but I want to say that I'd be leery of sending this, even if you describe it to the receiver. Sounds like the receiver is picky - this is absolutely his/her right, however, if you question the condition of your book, it will probably not ultimately meet their standards.  If it were me, I'd deny the request.

I would automatically decline such a condition. They have a right to their condition, but something like that too ambiguous. I would be leery of losing my credit because it wasn't "good" enough.

I too would refuse because this is a used book site, not a new book site!  I'd say I couldn't meet their conditions and if you wanted to (and the info was available) PM them and say that you couldn't have sent them a PM because you didn't know who they were and didn't want to accept because if you had to cancel it would not have let you repost the book.

which reminds me I need to make sure my user ID is in my condition notes!

I too would decline.  First she/he shouldn't ask for a PM for that purpose.  Now I put in my conditions that someone can PM me if they have a question, but not to determine if the deal is a go or not.  Second, it's too interpretive.  What's great to you may not be great to him/her.  Not worth the hassle or credit.  And thirdly, if she/he decides they don't want your book, who cancels?  Either she/he loses their place in line, which I doubt they'd be willing to do, or you have a book sitting around that someone else is waiting on and you have a potential credit that you can't get.  Too much hassle for me.  But that's just my opinion.

Thanks for your comments (and for verifying what I thought - there is no way to PM someone ahead of time unless you know who they are). I usually decline conditions that are vague or too stringent (the person who requested "new or like new" got a big No Way from me). But this book has been on my shelf for a long time (over a year!) so I was considering contacting the requester to describe the condition. I think I am just going to decline.