According to
this (adding C&GI, Stars Dance and Slow Down's updated sales) and
this, as well as their respective receipts threads in the chart section, Selena Gomez is officially the 2nd best selling Disney girl, surpassing Hilary Duff.
Miley has sold 16,050,000 albums and 37,812,000 singles in the US = 53,852,000
Selena has sold 2,752,000 albums in the US and 14,303,000 singles in the US = 17,055,000
Demi has sold 1,924,000 albums and 12,911,000 singles in the US = 14,845,000
Hilary has sold 8,137,000 albums and 3,066,000 singles in the US = 11,203,000
Congrats Selena and all of them.