I'm going to go with the more poetic side of things. Is it a
[Edit] There is a reason for the first line in my answer but due to popular request, I'll explain the reasoning behind my answer.
For some I bring the gift of life,
You can see it as the first step of a relation that leads to children or the sleeping beauty being awoken by the kiss of the prince.
For others I bring death instead,
The "kiss of death" or "Juda's kiss" signifying betrayal and for mafia for example, marking someone for death.
I've been known to mend relationships, Or to tear them apart with gossip and innuendo,
A kiss at the right time can fix things in a relation and kissing someone else can ruin a relationship. Especially if seen by someone.. gossip and innuendo will follow.
Some think of me as a wild rockstar, But I prefer to be a known as a simpleton,
This is stretching it a bit. Some see the first kiss as something special.. others just as a simple gesture.
I am often given, occasionally stolen, rarely sold, and never borrowed,
Giving a kiss is kind of self explanatory, stealing a kiss is sometimes used when one does not expect, selling.. I don't really want to go into details about this here. And borrowing.. didn't find a way of doing this.. Not in a decent way at least...