Both Alibaba.com and globalsource.com are online markets for product listings that enable potential buyers to select the best available options. There are pros and cons to both the platforms and depending on what you are looking for each may offer some unique advantages.
Alibaba offers a very wide platform for the listing of products across the entire range of sellers from different walks of life. Alibaba has free posting for small sellers, subscription-based service for niche clients (global brands), as well as a marketplace for exporters and importers. It is a relatively recent player in the market, though one of the biggest in the world and very profitable as well.
globalsource has been in business for a long time, since 1970. It offers a multi-channel platform to a bevy of trusted business groups. While globalsource is much smaller in e-commerce as compared to Alibaba, it offers "verified" sellers and provides concentrated services to medium and large enterprises, unlike the SME (small and medium enterprises) ecosystem that Alibaba intends to build.
So, while these two are not exactly competing for the same business, SME will benefit from Alibaba and medium and large enterprises will benefit more by working with globalsource.
A buyer of car parts of reasonable scale would be a medium enterprise and based on the "verified" seller feature and the more trustworthy nature, I would recommend globalsource.