The thing about Alibaba is that you're not doing business with Alibaba themselves, you're doing business with independent suppliers that are advertising in the Alibaba B2B directory. So my advice is that things like gold member status, while are OK to use as a bit of guide, they don't offer you any sort of protection.
The only way to fully protect yourself is to use a safe payment method, credit card ideally, PayPal if not. DO NOT use wire transfer services, that's the quickest way to wash your money away.
With sample lots. Yes, getting a sample lot is always a good idea. That said, you can never put all your faith in that alone either, so use it as a guide again.
It sounds like you are talking wholesale here, so you may want to consider using a sourcing service like Imex. They can verify legit suppliers for you to deal with and can save a lot of drama when you're talking large purchases.