I guess Online Markets sometimes offer products for prices that are sometimes half of what an Original Market can offer in price for the same products. I can't say you should fully trust it, though. My advice to you(like what I always advise myself when Purchasing something online) is always check the trust list of the seller both POSITIVE AND NEGATIVE, try reading users' reviews if possible and hope for the best(most purchasers/buyers mistake is neglecting the trust and going for the lower price).
Yes you're right but i think there isn't such thing like trust list on alibaba. If there were, it would be much better.