I sell some herbal weight loss drugs on ebay. Some I buy from New york supplier, and others from a chinese supplier.
In a worst case scenario, what if someone buys these from me and has a stroke or heart attack. Can I be held responsible? OR would the drug manufacturer? What if I add a disclaimer on the product page "like: please see your doctor before using" or something like that.

Yes, both you and the manufacturer can both be held liable if the formula is found to be the cause of the affliction.

You could be sued, you could definitely be sued. How confident do you feel about the quality of those herbs coming from China?

Unless you are licensed health care provider who would be allowed to sell the stuff on the street corner or off your front porch or from a store at the mall, you cannot do this in the US. Unlicensed individuals who have done this sort of trade in person have traditionally been prosecuted, and just because it is on EBay does not make it legal. In addition to peddling substances illegally, you would indeed be liable for untoward results. The fact that something is "herbal" does not mitigate the matter at all. If you were selling used coffee grounds "for weight loss", that would also be illegal. You are not allowed to diagnose, treat, or offer any item with the representation that it will achieve a medically-significant result, and a disclaimer is of no legal value for your protection.
There were a couple of old ladies in the Midwest who sold dimethylsulfoxide (DMSO) from their kitchens, saying it helped with arthritis. These ladies were selling the Kodak variety. (A pure, pharmaceutical grade of DMSO is marketed legitimately under the trade name Rimso 50, but what they were selling was the DMSO from Kodak that is used for film developing. Another form of DMSO is sold to various manufacturers as a de-greaser.) They were arrested, charged, and convicted in federal court.