In my case it depends on the buyer, how they write, what their attitude appears to be, their use of emoticons or lack thereof, the way they greet me or don’t greet me, their nickname (whether it’s funny, cryptic, real name, etc.)
Using all the above factors, I use either a professional or a friendly tone (or a mix of both) based on how I feel the buyer indirectly expects of me.
I know, I may be giving this too much thought for each of my clients, but it works miracles when you detect or sense the preferred expected tone of the buyer - it’s one of those little things that can turn a good experience into a great one, or make a buyer become a regular or a big tipper
P.S. whenever I can’t sense anything from the buyer’s side in regards to this, I use a professional tone as a fallback solution