Last time I was in Shanghai a couple years ago, not once did I see any H!P merchandise in any of the stores I went to. These included bookstores, video stores, music stores, etc. And believe me, I was looking pretty hard during that month I was there. That's not to say that there wasn't any Japanese merchandise, though. Mostly stuff like Avex and Hamazaki Ayumi though.
Saying they "already got market in China" would be like saying that they already have a market in the US, or Canada, or the UK. Sure, you might find individual fans who get their stuff imported, but there is no public exposure. Maybe it's different in HK or Taiwan, but I didn't see anything on the mainland.