Although you don't strictly speaking have to have a tax id# to sell on eBay, some suppliers are going to require it. As you mentioned, being in a sales tax-free state more or less obvious the purpose of resale certificates, so I doubt that New Hampshire has a whole office dedicated to keeping track of businesses from which it isn't going to be collecting revenue.
A tax id# is extremely easy to get -- just hit the IRS website (
Link hidden: Login to view) and follow the instructions.
Frank