Of course you can't.  People like you never can.  If you had anywhere near that level of insane success (15% sustained ROI is the equivalent of hitting 60% of your -110 bets), it would be extremely easy to document it. 

Let's do some math:

Let's say you started with a 1k bankroll (which is very very small when it comes to sports betting).  Let's say you stuck long-term with a VERY conservative 1%BR per bet.  You claimed to have made an average of 100 bets per month for the last 5 years.  That's 60 months.  If you only recalculated your bankroll once a month, that means that starting with a 1k bankroll you should have made a little over $3,800,000.  And remember those are using very conservative numbers.

Now why would someone like that waste their time selling their picks?