I usually just ignore offers sent via messages, but if I'm in a good mood I'll respond to the person, politely decline, thank them for looking, etc. I tend to find offers to be annoying, mostly because if I wanted offers on my items I would add the "best offer" feature (also free now, BTW). I run sales and sometimes lower prices (then still have sales) on most of my items, but some things I am okay with sitting on for awhile to get the price I am looking for.
Jman, regarding your seller, it seems like he was being a dick. Responding to your inquiry/offer with a higher price than his listing seems straight up snarky and kinda rude.