I have done business with sellers from both alibaba and tradekey. True there are a ton of scammers on both sites. However there are some legit ones too, depending on what you are out to buy. All of the ones that I used accepted paypal or credit cards. It's all a matter of doing your homework BEFORE you buy.
I always email with questions to see how fast they respond and what type of email service they are using. A legit company for example will not be using a 'free' service such as hotmail. Also see if they are willing to provide a sample,, things like that are invaluable to making an informed decision as to wether to do business with..
As with any transaction, you should do your homework first, don't be afraid to ask hard questions and only do business where you will have a solid recourse to reclaim your money if things go sour....