So, the websites who shows DB at 300-350 milllions are wrong?
you have to learn to DIG DEEPER and not blindly trust the number they put out, and of course this takes time.
For example your 350 million claim, here is
one of the the source
https://www.oricon.co.jp/news/61072/full/
The source claim 350 million worldwide, and the news is about Dragon Ball Evolution live action back in 2008 (there is also the same English article one).
Here is one claim that say Dragonball has
230 million copies worldwide back in 2013.
2013330
www.toei-anim.co.jp
From Toei Animation themselves, who worked closely with Shuiesha and was promoting Battle of the God movie.
In these two examples, Toei holds more weight since they work closely with Shuiesha.
The best source is probably from the publisher themselves [in the form of dust jacket cover advertisement, anime news, other news..]
and of course the newer the series, the better source we have (like majority of series from 2010+ are pretty accurate)