Thanks for your insight luckyfella.
Yes the warranty can be a problem. But if I can make a big margin for each product I sold, I could provide the warranty out of my own pocket. For example, Security Camera King sells Dahua's full D1 8 channel DVR for $369, I could get a similar model from China for just $80, reselling it at $200, I could get quite a nice profit. So it it's broken in a year, the customer just ships the unit back to me and I can send him a new one. Yes I may loose a couple dollars for each refund, but overall I think it's profitable.
I won't buy any inventory myself, the carrier I found can ship from China to the US within 5 ~ 8 days with a reasonable cost.
The only issue I have now is the technical support, which is time consuming and I'm sure Dahua won't give any support to those people who buy from me.