I'm guessing that number would be very high. But, I'm not sure if 300 million would be correct. I looked in Guiness to see if there was anything there, but, Queen was not mentioned.
But, I take that with a grain of salt, because most of the acts that are on the lists are ones that are still touring. And, there was no category for most albums sold.
But, there was mention of The eagles held the record for the greatest single album ever sold in the U.S. at 25 million. But that is so wrong when Michael Jackson's Thriller sold over 33 million.