There was a post tonight on the SL Answers forum from an avatar who had purchased $L321,000 from another avatar. She'd been told (I presume by that avatar) that he was "Risk API Positive", so she felt the transaction was approved by LL.
She was very upset when Linden Lab took those $L from her account, telling her they were fraudulent.
Here's the straight goods: There are no "Risk API Positive" avatars. The Linden Lab Exchange Risk API is a software tool furnished by LL to certain third party linden dollar exchange web sites. The API is not something that applies to individuals.
A popular money laundering scheme involves stealing a credit card, then using it to buy $L. Then these fraudulently-obtained $L are peddled on eBay or other venues. If you buy these "hot $L", Linden Lab will take them back out of your account, and assess you a 50% penalty besides.
All $L come from LL. The official exchange rate is always pretty close to $L250 = $1 USD. Anyone who offers to sell you $L at a rate significantly better than this should raise an immediate red flag.
NEVER buy $L from an individual. NEVER buy $L on eBay. There are legitimate third party linden dollar exchanges out there. Use them if you must...although LL will take no responsibility if you get taken, and the best price will nearly always be the official Linden Exchange on the SL web site.