I think that's fine - you're not leading with it, you've made the offer, and he said he'd negotiate but didn't give you a number. What else can you do? Aside from walk away.
There are not really any society-wide rules for polite negotiations, and in a major urban area you're possibly talking with people from other cultural backgrounds anyhow, so I don't think there really is a right or wrong way. I just do what works for me and is not too aggravating. If any pre-sale coms are aggravating or otherwise a red flag I just walk away. There is always a lot of other junk for sale instead.