For what it's worth, although I am old school, trained as a linguist and translator and worked as a writer in various capacities most of my life, my preference, when addressing an email message to a buyer, is to use "Hello" or "Hi", plus the person's first name if I have it.
I personally don't feel that most people expect the sort of formality in simple emails between buyer and seller that would be proper in a more formalized setting.
During my days representing arts organizations, I would never have addressed anyone outside my immediate colleagues in an email with "Hi" or "Hello". It's not so much a question of respect, but a question of context. In the context of my role as an eBay seller on the other hand, I feel that addressing a buyer in an email with "Dear Susan" for example, looks and sounds stiff and slightly remote.
On the other hand, "Hi" or "Hello" to start off an email to a buyer establishes an immediate and friendly approach that I think most people will respond to positively. I would add that this is especially true for Americans, who have become quite used to addressing each other in almost any context but formal letter-writing with "Hi" on a first-name basis. To address an American buyer with "Dear Jane" in an email might actually be taken to be rather snooty and cold.
In French, "Bonjour _________ [first name of person]" is the current equivalent for friendly seller/buyer emails, rather than the sort of staid types of address that are demanded in formal situations.
I think another factor that has played into the familiarization of English address in messages is the explosion of social media, where no one expects anyone to use any form of address at all. Who knows? The next, completely neutral, form of address in English may someday become @. 
By the way, rest assured, Google Translator seems to have this covered. I'm sure that if you use "Hi" or "Hello" at the beginning of an email to a buyer, it will be translated with the appropriately friendly but polite word in another language.
Since language is a dynamic thing, constantly evolving, it's a matter of shifting a little to bend with the wind. Here's an example: How many of us these days would refer to or introduce a 20-something young woman as "Miss so-and-so"? Fifty years ago you wouldn't dare do otherwise. However now that the word "Ms." has become so universally acceptable, and even preferable, using "Miss" would sound just a tad demeaning. That's language in motion. Go with the flow. 
