As stated on his eBay auction,
"Before the Olympic Games, EMS won't ship batteries by air for security reason. The items have to be shipped by UPS that cost more, but your item will be delivered faster. "
Which means the shipping cost will be higher which will eat into his profits(Maybe to the point of loss, depending on what his profit margin is.). This would influence him to hold-back stock until it becomes more profitable when the EMS lifts this restriction. Also, the supply shortage might be explained by either selling stock too quickly(perhaps due to recently rising market prices) and/or a true supply shortage that might have been influenced by the quakes in China(i.e., perhaps the powder manufacturer is based in South-Western Asia?).
It's just a guess.