Have you only sent emails? I gather from everything you've said, that you've emailed a couple times, and got no reply. Have you tracked the emails to see if they were actually opened? If yes, how many times have each email been opened. Have you emailed multiple people within the company? Have you actually called and spoke to someone?
1. Many times, I've emailed people and got no reply. Then, when I use some other email address (either for the same person or someone else in the company), I got a reply.
2. The number of times the email is opened or passed around may indicate interest. If there seems to be interest, don't completely move on from them. Send them follow up emails however often you choose (once a week, once a month, whatever).
3. Some sales require a lot of patience. I currently have one guy who promises me that he is very, very interested in one of my domains, but he's 'thinking about the purchase'. If another buyer comes by while he's thinking, I'll surely sell the name. But, I just have to wait right now; this same guy might be the eventual buyer or not. The ultimatum thing is a no, no in my opinion. You want to remain friendly. I've told the guy in my transaction, 'take your time, I know spending money requires a lot of thought.'
4. Many people will give you a more direct answer when/if you actually call and speak to them.....as a friend, not a salesperson.
Since you didn't give details about your actual communications with them, it's difficult to say if you should move on. Some successful salespeople never move on.
Anyway, that's my two cents.