You are right the NIV does say slave to sin... I was looking at the ESV. So which bible is right, was Paul sold under sin or sold as a slave to sin?
Have you ever sinned since you put God first in your heart?
As to your first point, sold under sin and slave to sin means the same thing.
As to your second point. You seem to have 'theological' understanding of scripture, but not a heartfelt knowledge of scripture. Strangely, this happens to many. Theologically they say much of the right things about grace, but lack something else that is needed. At it is the theological mindset that makes people refuse to accept specific future blessings for the nation of Israel incidentally.
This is the spiritual reality:
Paul spoke to the romans in ch6 saying they used to be slaves to sin before they came to Christ(sin dominated their lives, they were sins slave) but by following the Gospel message he gave them they were set free from sin(understand he is not saying they are sinless, but free from sin dominating their lives/they are its slaves in that context) and they were being led into holiness(leading evermore holy lives) The Paul of rom 7 could not grow to live an evermore holy life. He was controlled by the sin. He could not do what he wanted to do but what he hated to do he kept on doing etc
There are sins that by their very nature will stop you crossing over from being a slave of sin to a slave of righteousness(being led into holiness) these have to be dealt with for you to cross over from one state to another. I could go into this further, but you will not accept it, for you it would not understand it(no offence meant)
Of course I commit sin, no Christian who had been born again of the Spirit could truthfully state they never sin, the standard set by the law is far too high for anyone to perfectly attain to. However there is a huge difference between being a slave to sin(sin masters and dominates your life) and you living an ever more holy life(being led into righteousness) where you see much victory over sin.