The 2012 Uncirculated Gold Buffalos were first offered to Mint Authorized Dealers sometime in the first week of March 2012. The coins were offered around the same time frame as they were in the previous year or in 2011.
After the coins were purchased by the Mint Authorized Dealers they were then resold to collecotrs and other dealers. By April 2012, the coins were already entering the market.