This just happened to me. I tried 3 times to send a message, with the same broken Google thing appearing each time. But... the message went through. I always tick the "Send a copy to my email address" thing so I have a permanent record, and I found I have 3 emails containing my message. So even though it looks like it didn't work, make sure you tick that box and then check your email to find out.
Of course, I then had to send a 4th email to the seller apologising, which was particularly annoying after messaging them to dispute a problem with a recent purchase.
