Don't be shocked, this is old news. Standard practice in the regular bike industry as well. If you buy from a legitimate dealer, or an established direct to consumer business (radrover or voltbike for instance), you'll get support for your money spent.
Also, the prices on Alibaba may be for substantially different parts on the bike (including motor and battery size as well). When you build something part for part, the difference may be much less.
If you are thinking Alibaba, you're better off going for with a bike from bikes direct and a kit from Luna cycles. If you're not comfortable with a kit, skip Alibaba because you're not ready to deal with issues that will arise without support for the purchase. I do a lot of my own work and maintenance but would never buy Alibaba...you're dealing with a factory, how much time do you think they'll spend getting an order of a single bike right for you?