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I am asking this question because my xBPDGF stole money from me saying she had cancer when she didnt. I did give her money blindly trusting her for 3 years. Whenever we met she used to steal something or the other from my apartment things like Apple iphone charger, unopened gifts, books, belts etc etc. Thought of knowing whether its a common thing and if it is why do they do it.

It's not a diagnostic criterion of BPD, but it could fall under Impulsivity,  and stealing could cover up other behaviors. 

I wouldn't say my dBPD mother steals,  but she plays the role of the poor widow on social security so people rescue her financially.  Objectively,  she's a widow on SS. In her situation,  I could get by fine on her fixed income.  She doesn't see this,  and her feelings are her reality.  I see a lot wrong with it,  taking advantage of people under false pretenses.  For decades,  however,  this is how she's survived. 

It's likely a survival mechanism.  As Christine Ann Lawson says in Understanding The Borderline Mother,  "lying feels like survival." Stealing may fall under a similar rubric. 

My exgf didn't come right out and steal but she loved to hold on to my things to control me. If she were upset she would not give my things back to me.

I know she stole from others though, she used to sneak into her brothers homes and go through their med cabinets and take the "good stuff" whatever that was.

She stole my sunglasses one time, when I went to get them she told me our son broke them, called me "bad daddy" too. Funny thing was, he wasn't even talking at that time in his life.

Oh, how I miss those days, walking down memory lane... .hmmm yes... .she's insane

Hey Jerry
I can relate to that... .they live on their lies and play the victim card always.

Maybe your ex have also traits of Antisocial Personality Disorder

Not sure but i care less... .Enough of this nonsense going on in my life since 3 years. She drained me emotionally and financially.

I think a lot of this ties in to their constant need to "win" or "get one over" on someone.

My BPD ex gf screwed me over financially pretty badly and I could tell she got great pleasure in doing so. I never did anything but give and give and give, always paying for everything and easily spent tens of thousands of dollars on her throughout our 1.5yr relationship yet she still felt the need to put me through the ringer financially when she discarded me.

I second this. Same happened to me.

Amen to that

I recall one year my employer gave me my bonus twice - two deposits in my bank account instead f one. My uBPDxw got excited and suggested I keep it. It was 5 figures and nothing to sneeze at. I lost a lot of respect for her that day and that was well before everything that happened at the end of our marriage. In hindsight, it should have been a warning.