I actually did this once, because I was interested in a particular house in a particular neighborhood that had particular exterior features. It was a very classy craftsman fixer-upper with a view, extra-sized lot, and that was the perfect square footage and appeared to need some restoration. Perfect, but not for sale!
I would not want to buy myself a compromise only to see this one come on the market months later. So when an agent begged to be my buyer's agent, I said "sure, send a letter to the owners of this particular house." She actually knocked on the door and asked. The answer was they were not interested in selling. I dropped the buyer's agent and bought a compromise a few months later. (what an asshole, right? ;)
Then again, you're talking about a townhouse, which is at least very similar to its neighbors. Probably in this case, the buyer (if they exist) are canvassing for a good deal - for example, a seller in a financially desperate situation. Or maybe they just hope to avoid a bidding war.
I would quote them a firm fixed price that you'd be thrilled with and leave it at that. They'll either run or ask to take a look.