When buying from an agent, you usually have the choice between Taobao/Tmall, Weidian and 1688.
Taobao and Weidian are equally good. Taobao is more selective with it's sellers, but that can also be a downside. They have minimal fees, and the prices are comparable.
1688 tends to require bulk orders, some agents allow you to place an order for a single item and hope that the seller accepts it, but expect complications if you order from there. Especially as a beginner, I'd try to stay away - unless you find a great deal of course.
Among all the items I bought I don't think a single one has been from Tmall, that doesn't make it a bad platform, but it usually doesn't play an important role.