Is it true that our internet history and activity is being sold?
can you explain how and why this is happening?

At this moment our current Website search history and activity are not sold flat out.
What is currently in effect is the allocation of information to the government with this information being shared in extreme cases.
What was passed by congress earlier this year removes the justification of requiring the information. This bill has been passed by congress but keep in mind the supreme court must find it constitutional.
In the mean time us apps like RED onion and Tor browsers if your feeling paranoid, they keep any internet history away from your internet companies.