Loads of websites aren't selling a thing. In that case, I think an email or contact form for email is sufficient, as long as you're actually responsive. Google doesn't even offer that much on their informational pages, right?
And neither does Amazon, so when you say you wouldn't buy from a site that doesn't list its address and all that, realize Amazon is like that. They have forms for emailing them and/or getting them to call you, but if you want to mail them something at their physical address you'll have to go to another website to get that address.