[juven14]

You could do this with a module, there are already hooks in place when the listing is modified.

You'd just need to add in to that hooks function to email if this field is changed. On the template you'd need a new hidden field to record the inital value and then compare that field's input against it, if it is different send email otherwise do nothing.

Edit: Sorry, I didn't see that this was in the 3.0 forum, my above statement only applies to 3.1 versions.