I'm not sure how it works in the US, but here in the UK we have the Data Protection Act. Under that you are not allowed to store people's data without their permission. If you do store it, you are supposed to register with the information commissioner, you have to keep it up to date, accurate, only keep it for a specified purpose, and only keep it for as long as it is needed. So if you're collecting information about your buyers you should really ask their permission first.
If you are sending unsolicited e-mails through Ebay and the buyer thinks that these are unwelcome they could be forwarded onto eBay who could impose sanctions. Unsolicited e-mails direct to buyer's own e-mail addresses may not be welcome and I would be suprised if these resulted in many sales. I think the safest thing to do is to encourage people to add you to their favourite sellers and send out newsletters through eBay.
Just my opion, you might want to ignore it, but you said you wanted a number of responses.