Well - as much as we'd like to say all that, its probably a bit too early to get that defensive. In the end, you want a sale. People are stubborn and if they feel to pissed off enough about something they'll refuse to buy it on what they say is "principle" - despite it being to their own detriment.
I would simply stand firm on the price and ignore their comments. Or if you're really pissed, you could even respond with a "your response sounds like a thinly veiled (and ill-advised) threat, which I will not respond to" and simply close the negotiation. They'll eventually realized they screwed up because seriously, your price isn't unreasonable at all.
It just sucks that you can't have much of a real conversation due to the platform limitations.
But remember, you control the domain and you can control the content on it. Use the domain landing page to encourage any interested parties to contact you directly. They'll eventually see it and contact you if they were serious at all. You can even use the landing page to show comparable sales or domain valuations backing up your price, maybe educating them a little about domain values. That might soften them up as they understand more. Even though you know you're putting up that content specifically for that buyer, it can just be good general information too.
If they do contact you with a more conciliatory tone, then stick firm to your price (or negotiate if you really want to) and act sympathetic to a start-up not having the funds for a premium domain and then offer them a lease or payment plan.
But if they contact you with even a hint of more bullshit, then give them the full earful like above.
And just for kicks, look up the TM application: http://www.cipo.ic.gc.ca/app/opic-cipo/trdmrks/srch/home?lang=eng
Just don't use the TM info to contact them directly as they'll try and use it against you in a CDRP.