if you are sending out emails to sell domains and get a response of how much, you should have a figure in mind to give them. Since you are initiating the sale I don't think it's fair to tell them to make an offer. And if you do ask them to make an offer, you'll probably get offers of $100 or less most of the time.
You could make your asking price based on how well the domain fits the company, do they own the .net version and you have the .com, are they a big company, does the name have traffic, how valuable would the name be to their business, etc.
I have to admit that sometimes I get emails from people trying to sell me domains that have no interest to me at all - these are like spam. Other times I get offers for a .com when I already own the .net or .ca version, and have actually bought a few like that for $100 or less. If I can see why they are offering me the domain (ie: they know I would likely have an interest in it), then they have done their homework and it's not really spam.
side story: a few weeks ago I was offered a .com domain out of the blue by a catching company because they saw I had the .net. they were asking $75 and though I was interested I didn't act. looked up the domain yesterday and they had let it go, so I got it for reg fee at godaddy in the end!