I believe that Alibaba is popular but there is alot of scammers on that website because they dont offer escrow. However, Alibaba launched their escrow site which is called aliexpress! This is a great site to buy from if your a re-seller. The other sites that are popular that have escrow is DHgate. They also have a large list of quality products, at very good prices. MadeInChina is another I have used which is safe and uses escrow.
Sometimes these sites mentioned don't seem to have really specific product ranges. In this case I would look to Global Sources. But beware the dont have an escrow service so to speak, so search for the companys that are audited verified and manufacturer verified, and build a relationship with the seller via Skype or Msn. They usually sell in BULK only. Hope this helps