Is there an unwritten shorthand code for buyers to leave feedback?

No, there isn't.

There are places where someone might use an acronym, in an ad or in feedback, but on the whole, they would be acronyms which are readily recognised by the majority of people.

The whole purpose of writing is to convey a message. DR generally means Doctor if it is in front of a name. On its own, it is pretty meaningless to most people, so in effect your buyer failed-he or she did not convey any message at all. But they gave you a green dot, so that is the message others will take from it.