Upload & Sell: Off
What do you mean by "purchase the image"? Are they asking for a transfer of copyright? If so, you're the only one who can decide on the value of that particular piece of intellectual property.
Hey John, they did want the copyright, but I didn't feel $300 was sufficient to give up rights to the image.
You're right, I have to come to decide what my work is worth after looking into the market. I ended up giving them a non-exclusive license to the image and will be scheduling shoot for other images they're looking for.