There are ways to find the buyer name, but in general it’s not useful. Personalizing your message isn’t nearly as important as making sure they know that you understood their request. Don’t worry about what to call them, just start with “Hello.” (Don’t use Sir or anything similar.) You need to send your buyer request with enough information to get the buyer interested enough to contact YOU. If they are asking you to send files or something, you can tell them that you will do so when they contact you, but send them a great pitch.
Even if the PC browser displayed the buyer username clearly, if you send that buyer an inbox message for ANY reason, they have the right to report you for spam. You could get your account blocked.
If you really want to find the buyer name, I’m sure you can find a way and there are people here who will tell you how. You shouldn’t need it, though, and the lack of it is therefore not a bug.