I'd be pissed. You are essentially asking the book at the price that's OK for you and the seller has the right to assume that if he accepts that price without countering, it's a done deal.
When I send a request with a specific price in the PM, if accepted, I always consume the deal.
If I am on the fence, I send a PM with the following question: "What's the best you can do on this book?" If the price I hear back is not OK for me, I thank the seller and might pass on the book (or counter, which I rarely would do in that instance) as I never indicated that I will accept the new price offered.
I think to avoid any confusion and hard feelings either way, that the proper approach is for a buyer to do one of two things:
If the buyer is serious about the item in question, the phrase should be "If you'll sell for $X, I'll take it." This is an offer to buy and the seller can then accept or reject the offer in a reasonable time.
If the buyer is just fishing, then the phrase should be as Scrooge says "What's the best you can do on the item?" This is an inquiry asking for an offer, rather than an offer.
This should work for everybody