I've got a few of those emails but I'm not ready to sell. And from what I can tell they will ask you for a lot of details on traffic, earnings, etc and sorta lowball you. I guess it depends on who's inquiring.
I never feel comfortable giving out my traffic & earnings stats to total strangers, I only made exceptions to people I got to know over time and that I knew were serious about it. My rule is the one who inquires first need to make the first offer. You know at least after that if they are on the same ball park.