I'm just using eBay's selling manager pro automated e-mails. A bit lazy perhaps, but hey that's what automation's about isnt it!?
So basically they just get the same e-mails all eBay users get when they make a purchase, but I have included my logo and a personal note in each one inviting them to look at other similar listings, combine postage, leave feedback or contact me. I haven't set up my site yet but when I do I'll also include that in my e-mails. (asking them to tell you when their item arrives safely seems to generate feedback too)
I had a go at using Auctiva's selling tools including automated e-mail, but personally it wasn't for me.
So that's the post-purchase e-mails. If someone contacts me with a question I simply hit "respond" in eBay's built in messaging and try to keep that e-mail as short as possible. I hate writing long e-mails and I think most buyers hate reading them lol.
(I seem to like writing long forum posts tho... o_O )