I am new to e-mail marketing, and thought someone here might be able to clarify for me as to what is considered spam.

I went to the Spamhause website, and read about it there, and well, I'm still confused.

I have had an Ebay account for quite some time now, and have decided to have my own web site selling, exactly the same items as I do on Ebay. Since I have a list of about 400 customers from Ebay, I would like to let them know about the new web site and provide a link to it in the e-mail. Nothing more, nothing less.

I read about the part on Spamhause "it's not about content it's about consent", but still cannot understand how you are to get anyone to "opt in", if they have never seen or heard of the web site in the first place ?
As I have few visitors to the site at present, having a subscribe button there would be pointless.
So my question is two fold :-

If the recipient of the e-mail has purchased from me before (albeit from Ebay), and therefore has had e-mail contact from me before, can this be considered an opt-in to send them my invite e-mail ?

If the answer is no, then my next question is :-

As long as you are not actually selling anything in that e-mail, i.e. you are simply informing them or the existance of the web site, is this still spam ?

I do not intend the send these invite e-mails more than once, per e-mail address.

Your clarity on this would both helpful and appreciated.